[Sanitizers] Check pthread_setcancel{state|type} interceptor arguments for != nullptr.

Summary:
According to man, pthread_setcancelstate's oldstate and
pthread_setcanceltype's oldtype parameters can be nullptr.
Check these parameters for != nullptr before attempting to
access their shadow memory.

Reviewers: dvyukov

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D39626

llvm-svn: 317494