CppEditor: Fix possible null pointer access

Fixes: QTCREATORBUG-29484
Change-Id: I047ea13e6808902a3b0fb94b9eacdd48e411a22a
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cpprefactoringchanges.cpp

@@ -124,6 +124,9 @@ bool CppRefactoringFile::isCursorOn(unsigned tokenIndex) const
 
 bool CppRefactoringFile::isCursorOn(const AST *ast) const
 {
+    if (!ast)
+        return false;
+
     QTextCursor tc = cursor();
     int cursorBegin = tc.selectionStart();