Check for null pointers before calling the Stmt Profiler

Some calls from OMPClauseProfiler were calling the Stmt Profiler with null
pointers, but the profiler can only handle non-null pointers.  Add an assert
to the VisitStmt for valid pointers, and check all calls from OMPClauseProfiler
to be non-null pointers.

llvm-svn: 272368