[sanitizer_common] Fix potential null dereference in dlopen interceptor (#74645)

The test_only_replace_dlopen_main_program flag
(introduced in
https://github.com/llvm/llvm-project/commit/0be4c6b9483594494051e8f1f67afc2b516270ca

)
will cause internal_strcmp to dereference NULL if DlAddrSelfFName()
returns NULL (which happens in very rare cases). This patch adds a
null pointer check.

Co-authored-by: Thurston Dang <thurston@google.com>