[analyzer] Fixup for r162935 as per Jordan's review.

Thanks for catching this!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162949 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/StaticAnalyzer/Checkers/BasicObjCFoundationChecks.cpp b/lib/StaticAnalyzer/Checkers/BasicObjCFoundationChecks.cpp
index e5fbb83..cfb35a4 100644
--- a/lib/StaticAnalyzer/Checkers/BasicObjCFoundationChecks.cpp
+++ b/lib/StaticAnalyzer/Checkers/BasicObjCFoundationChecks.cpp
@@ -736,9 +736,9 @@
                                     ProgramStateRef State,
                                     CheckerContext &C) {
   SVal Val = State->getSVal(NonNullExpr, C.getLocationContext());
-  if (!isa<DefinedOrUnknownSVal>(Val))
-    return State;
-  return State->assume(cast<DefinedOrUnknownSVal>(Val), true);
+  if (DefinedOrUnknownSVal *DV = dyn_cast<DefinedOrUnknownSVal>(&Val))
+    return State->assume(cast<DefinedOrUnknownSVal>(*DV), true);
+  return State;
 }
 
 void ObjCNonNilReturnValueChecker::checkPostObjCMessage(const ObjCMethodCall &M,