[libcxxabi] [test] Avoid mingw warnings about missing a return statement

With current versions of mingw-w64 headers, code following
assert(false) isn't considered unreachable - thus add a dummy
"return nullptr;", to avoid warnings (treated as errors) for a
missing return statement.

The root cause does get fixed further upstream in mingw-w64 in
mingw-w64/mingw-w64@1690994
though.

Differential Revision: https://reviews.llvm.org/D147860

libcxxabi/test/cxa_vec_new_overflow_PR41395.pass.cpp

@@ -18,7 +18,7 @@
 void dummy_ctor(void*) { assert(false && "should not be called"); }
 void dummy_dtor(void*) { assert(false && "should not be called"); }
 
-void *dummy_alloc(size_t) { assert(false && "should not be called"); }
+void *dummy_alloc(size_t) { assert(false && "should not be called"); return nullptr; }
 void dummy_dealloc(void*) { assert(false && "should not be called"); }
 void dummy_dealloc_sized(void*, size_t) { assert(false && "should not be called"); }