sanitizer: fix crash with textdomain(NULL) interceptor

Summary:
The textdomain function accepts a NULL parameter (and should then return the
current message domain). Add a check for this and include ASAN tests.

Link: https://github.com/google/sanitizers/issues/787

Reviewers: m.guseva, kcc

Reviewed By: kcc

Subscribers: kubamracek

Differential Revision: https://reviews.llvm.org/D32318

llvm-svn: 300924