[asan] Allow getpwnam(NULL) for binary compatibility

Calling getpwnam(NULL) is probably a bug, but at least on Darwin, such a call succeeds without segfaulting. I have some existing code that relies on that. To maintain binary compatibility, ASan should also survive a call to getpwnam with NULL.

Differential Revision: https://reviews.llvm.org/D40052

llvm-svn: 319344