cad/brlcad: fix null pointer accesses during build

During an exp-run for llvm 12 (see bug 255570), it turned out that
cad/brlcad does not build with clang 12.0.0:

[ 99% 4379/4403] cd /wrkdirs/usr/ports/cad/brlcad/work/brlcad-7.30.2/db/nist && /wrkdirs/usr/ports/cad/brlcad/work/brlcad-7.30.2/bin/step-g -O /wrkdirs/usr/ports/cad/brlcad/work/brlcad-7.30.2/share/db/nist/NIST_MBE_PMI_11.g /wrkdirs/usr/ports/cad/brlcad/work/brlcad-7.30.2/db/nist/NIST_MBE_PMI_11.stp > /wrkdirs/usr/ports/cad/brlcad/work/brlcad-7.30.2/db/nist/NIST_MBE_PMI_11.log 2>&1
FAILED: share/db/nist/NIST_MBE_PMI_11.g

What happens is that the step-g intermediate program segfaults, because
it attempts to access a null pointer. Valgrind shows:

Reading Data from /wrkdirs/share/dim/ports/cad/brlcad/work/brlcad-7.30.2/db/nist/NIST_MBE_PMI_11.stp...

HEADER read:
==24919== Invalid read of size 4
==24919==    at 0x1337BA10: EntList::firstNot(JoinType) (entlist.cc:39)
==24919==    by 0x1337C93E: nextNot (complexSupport.h:185)
==24919==    by 0x1337C93E: AndList::matchNonORs(EntNode*) (non-ors.cc:135)
==24919==    by 0x1337B77C: ComplexList::matches(EntNode*) (complexlist.cc:176)
==24919==    by 0x1337B36A: ComplexCollect::supports(EntNode*) const (collect.cc:140)
==24919==    by 0x1335FA5A: STEPcomplex::Initialize(char const**, char const*) (STEPcomplex.cc:126)
==24919==    by 0x1335F774: STEPcomplex::STEPcomplex(Registry*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const**, int, char const*) (STEPcomplex.cc:33)
==24919==    by 0x1331842E: STEPfile::CreateSubSuperInstance(std::__1::basic_istream<char, std::__1::char_traits<char> >&, int, ErrorDescriptor&) (STEPfile.cc:1048)
==24919==    by 0x13315E15: STEPfile::CreateInstance(std::__1::basic_istream<char, std::__1::char_traits<char> >&, std::__1::basic_ostream<char, std::__1::char_traits<char> >&) (STEPfile.cc:833)
==24919==    by 0x133158B1: STEPfile::ReadData1(std::__1::basic_istream<char, std::__1::char_traits<char> >&) (STEPfile.cc:502)
==24919==    by 0x13319EA8: STEPfile::AppendFile(std::__1::basic_istream<char, std::__1::char_traits<char> >*, bool) (STEPfile.cc:1674)
==24919==    by 0x1331C984: STEPfile::ReadExchangeFile(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, bool) (STEPfile.inline.cc:119)
==24919==    by 0x3AFDCE: STEPWrapper::load(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) (STEPWrapper.cpp:1300)
==24919==  Address 0x8 is not stack'd, malloc'd or (recently) free'd
==24919==
==24919==
==24919== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==24919==  Access not within mapped region at address 0x8
==24919==    at 0x1337BA10: EntList::firstNot(JoinType) (entlist.cc:39)
==24919==    by 0x1337C93E: nextNot (complexSupport.h:185)
==24919==    by 0x1337C93E: AndList::matchNonORs(EntNode*) (non-ors.cc:135)
==24919==    by 0x1337B77C: ComplexList::matches(EntNode*) (complexlist.cc:176)
==24919==    by 0x1337B36A: ComplexCollect::supports(EntNode*) const (collect.cc:140)
==24919==    by 0x1335FA5A: STEPcomplex::Initialize(char const**, char const*) (STEPcomplex.cc:126)
==24919==    by 0x1335F774: STEPcomplex::STEPcomplex(Registry*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const**, int, char const*) (STEPcomplex.cc:33)
==24919==    by 0x1331842E: STEPfile::CreateSubSuperInstance(std::__1::basic_istream<char, std::__1::char_traits<char> >&, int, ErrorDescriptor&) (STEPfile.cc:1048)
==24919==    by 0x13315E15: STEPfile::CreateInstance(std::__1::basic_istream<char, std::__1::char_traits<char> >&, std::__1::basic_ostream<char, std::__1::char_traits<char> >&) (STEPfile.cc:833)
==24919==    by 0x133158B1: STEPfile::ReadData1(std::__1::basic_istream<char, std::__1::char_traits<char> >&) (STEPfile.cc:502)
==24919==    by 0x13319EA8: STEPfile::AppendFile(std::__1::basic_istream<char, std::__1::char_traits<char> >*, bool) (STEPfile.cc:1674)
==24919==    by 0x1331C984: STEPfile::ReadExchangeFile(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, bool) (STEPfile.inline.cc:119)
==24919==    by 0x3AFDCE: STEPWrapper::load(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) (STEPWrapper.cpp:1300)
==24919==  If you believe this happened as a result of a stack
==24919==  overflow in your program's main thread (unlikely but
==24919==  possible), you can try to increase the size of the
==24919==  main thread stack using the --main-stacksize= flag.
==24919==  The main thread stack size used in this run was 16777216.

To fix this, add null pointer checks to EntList::firstNot() and various
other EntList functions.

Approved by:	erik@brlcad.org (maintainer)
PR:		256166
MFH:		2021Q2

cad/brlcad/files/patch-src_other_stepcode_src_clstepcore_complexSupport.h

@@ -0,0 +1,26 @@
+--- src/other/stepcode/src/clstepcore/complexSupport.h.orig	2020-03-12 15:32:36 UTC
++++ src/other/stepcode/src/clstepcore/complexSupport.h
+@@ -182,19 +182,19 @@ class SC_CORE_EXPORT EntList {
+         // but all we need.
+         EntList * firstNot( JoinType );
+         EntList * nextNot( JoinType j ) {
+-            return next->firstNot( j );
++            return next == NULL ? NULL : next->firstNot( j );
+         }
+         EntList * firstWanted( MatchType );
+         EntList * nextWanted( MatchType mat ) {
+-            return next->firstWanted( mat );
++            return next == NULL ? NULL : next->firstWanted( mat );
+         }
+         EntList * lastNot( JoinType );
+         EntList * prevNot( JoinType j ) {
+-            return prev->lastNot( j );
++            return prev == NULL ? NULL : prev->lastNot( j );
+         }
+         EntList * lastWanted( MatchType );
+         EntList * prevWanted( MatchType mat ) {
+-            return prev->lastWanted( mat );
++            return prev == NULL ? NULL : prev->lastWanted( mat );
+         }
+
+         JoinType join;

cad/brlcad/files/patch-src_other_stepcode_src_exp2cxx_complexSupport.h

@@ -0,0 +1,26 @@
+--- src/other/stepcode/src/exp2cxx/complexSupport.h.orig	2020-03-12 15:32:37 UTC
++++ src/other/stepcode/src/exp2cxx/complexSupport.h
+@@ -162,19 +162,19 @@ class EntList {
+         // but all we need.
+         EntList * firstNot( JoinType );
+         EntList * nextNot( JoinType j ) {
+-            return next->firstNot( j );
++            return next == NULL ? NULL : next->firstNot( j );
+         }
+         EntList * firstWanted( MatchType );
+         EntList * nextWanted( MatchType mat ) {
+-            return next->firstWanted( mat );
++            return next == NULL ? NULL : next->firstWanted( mat );
+         }
+         EntList * lastNot( JoinType );
+         EntList * prevNot( JoinType j ) {
+-            return prev->lastNot( j );
++            return prev == NULL ? NULL : prev->lastNot( j );
+         }
+         EntList * lastWanted( MatchType );
+         EntList * prevWanted( MatchType mat ) {
+-            return prev->lastWanted( mat );
++            return prev == NULL ? NULL : prev->lastWanted( mat );
+         }
+
+         JoinType join;