[asan] Zero initialize sem_t in sem_init.

Old version of sem_init (GLIBC_2.0) fails to initialize parts of
sem_t that are used in sem_timedwait. This is fixed in GLIBC_2.1,
but since ASan interceptors downgrade sem_* to the oldest available
version, this can introduce bugs that are only present in sanitized
build. Workaround by zero-initializing sem_t in sem_init.

llvm-svn: 250113