Thread safety analysis: Fix crash for function pointers

For function pointers, the FunctionDecl of the callee is unknown, so
getDirectCallee will return nullptr. We have to catch that case to avoid
crashing. We assume there is no attribute then.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@342519 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed