libclang: fix a crash in clang_disposeDiagnosticSet when it is passed a NULL argument


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201236 91177308-0d34-0410-b5e6-96231b3b80d8
Arcanist failed to commit the two clang test corrections that should have been in r201237.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201238 91177308-0d34-0410-b5e6-96231b3b80d8