[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
https://github.com/mingw-w64/mingw-w64/commit/1690994f515910a31b9fb7c7bd3a52d4ba987abe
though.

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