SLUDGE: Fix build with Clang >= 11.0.0

[knobix]

Clang 11 and newer check the types of return values more strictly and
generate an error if there is a discrepancy.

However the "bool loadGame()" function, returns the value "NULL", if
loading of a savegame was unsucessful, whereas Clang expects a return
value of type "bool".

Remedy the issue by using "false" as return value instead of "NULL".

[knobix]

On FreeBSD 13.0 (amd64 or i386), which has Clang 11.0.1, the build with --enable-all-engines for the pre-release 2.3.0 fails with following error:

engines/sludge/saveload.cpp:195:10: error: cannot initialize return object of type 'bool' with an rvalue of type 'nullptr_t'
                return NULL;
                       ^~~~
/usr/include/sys/_null.h:37:14: note: expanded from macro 'NULL'
#define NULL    nullptr
                ^~~~~~~
1 error generated.
gmake[2]: *** [Makefile.common:164: engines/sludge/saveload.o] Error 1

This issue does not appear with FreeBSD 11.4 and 12.2, which have Clang 10.0.0 and 10.0.1, respectively.

[bluegr]

Nice catch, thanks! Merging