only notify callbacks if they exist.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62334 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Lex/Pragma.cpp

@@ -400,7 +400,8 @@ void Preprocessor::HandlePragmaComment(Token &Tok) {
   }
   
   // If the pragma is lexically sound, notify any interested PPCallbacks.
-  Callbacks->PragmaComment(CommentLoc, II, ArgumentString);
+  if (Callbacks)
+    Callbacks->PragmaComment(CommentLoc, II, ArgumentString);
 }