[dfsan] Don't clear shadow on dlopen(NULL, flags)

This ports msan https://reviews.llvm.org/D14795 to dfsan.
dfsan, like msan, clears shadow for globals in a newly opened DSO in
case the DSO occupies the address of a previously labeled/poisoned area.
The operation should not happen on the main executable.

In addition, for a DT_EXEC executable, l_addr is zero and will lead to a
null pointer dereference in ForEachMappedRegion.

compiler-rt/lib/dfsan/dfsan_custom.cpp

@@ -842,7 +842,7 @@ __dfsw_dlopen(const char *filename, int flag, dfsan_label filename_label,
               dfsan_label flag_label, dfsan_label *ret_label) {
   void *handle = dlopen(filename, flag);
   link_map *map = GET_LINK_MAP_BY_DLOPEN_HANDLE(handle);
-  if (map)
+  if (filename && map)
     ForEachMappedRegion(map, dfsan_set_zero_label);
   *ret_label = 0;
   return handle;

compiler-rt/test/dfsan/custom.cpp

@@ -1,7 +1,7 @@
 // RUN: %clang_dfsan %s -o %t && DFSAN_OPTIONS="strict_data_dependencies=0" %run %t
 // RUN: %clang_dfsan -DSTRICT_DATA_DEPENDENCIES %s -o %t && %run %t
 // RUN: %clang_dfsan -DORIGIN_TRACKING -mllvm -dfsan-track-origins=1 -mllvm -dfsan-combine-pointer-labels-on-load=false -DSTRICT_DATA_DEPENDENCIES %s -o %t && %run %t
-// RUN: %clang_dfsan -DORIGIN_TRACKING -mllvm -dfsan-track-origins=1 -mllvm -dfsan-combine-pointer-labels-on-load=false %s -o %t && DFSAN_OPTIONS="strict_data_dependencies=0" %run %t
+// RUN: %clang_dfsan -DORIGIN_TRACKING -mllvm -dfsan-track-origins=1 -mllvm -dfsan-combine-pointer-labels-on-load=false -no-pie %s -o %t && DFSAN_OPTIONS="strict_data_dependencies=0" %run %t
 //
 // Tests custom implementations of various glibc functions.