[libcxx] Fix thread join.pass.cpp segfault after r271475

Some pthread implementations do not like being called pthead_join()
with the pthread_t argument set to 0, and causes a segfault. This
patch fixes this issue by validating the pthread_t argument before
invoking pthread_join().

NFC.

Differential revision: http://reviews.llvm.org/D20929

Change-Id: Ief817c57bd0e1f43cbaa03061e02417d6a180c38
Reviewers: EricWF
llvm-svn: 271634