[C API] Prevent nullptr dereferences in C API for counting attributes.

See https://reviews.llvm.org/D26392

Patch by @maleadt

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287044 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed