Check for NULL before traversing the isa<> type hierarchy checking with a

NULL-based reference.

Note: Encountered this a few times on Tiger + gcc 4.0.1. Might just be a
platform-specific compiler issue, but it's good defensive programming in any
case.

llvm-svn: 59890

include/llvm/Support/Casting.h

@@ -98,7 +98,7 @@ template<class FromCl>
 struct isa_impl_cl<FromCl*> {
   template<class ToCl>
   static bool isa(FromCl *Val) {
-    return isa_impl_cl<FromCl>::template isa<ToCl>(*Val);
+    return (Val != 0 && isa_impl_cl<FromCl>::template isa<ToCl>(*Val));
   }
 };