Add extra null check in clang_disposeString().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@93328 91177308-0d34-0410-b5e6-96231b3b80d8

tools/CIndex/CIndex.cpp

@@ -1065,7 +1065,7 @@ const char *clang_getCString(CXString string) {
 }
 
 void clang_disposeString(CXString string) {
-  if (string.MustFreeString)
+  if (string.MustFreeString && string.Spelling)
     free((void*)string.Spelling);
 }