[Analyzer] Fix introduced regression: properly check for nullable attribute.

llvm-svn: 315492
diff --git a/clang/lib/StaticAnalyzer/Checkers/NonnullStringConstantsChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/NonnullStringConstantsChecker.cpp
index 47b3304..516544b 100644
--- a/clang/lib/StaticAnalyzer/Checkers/NonnullStringConstantsChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/NonnullStringConstantsChecker.cpp
@@ -122,7 +122,8 @@
     return true;
 
   if (auto *T = dyn_cast<ObjCObjectPointerType>(Ty)) {
-    return T->getInterfaceDecl()->getIdentifier() == NSStringII;
+    return T->getInterfaceDecl() &&
+      T->getInterfaceDecl()->getIdentifier() == NSStringII;
   } else if (auto *T = dyn_cast<TypedefType>(Ty)) {
     return T->getDecl()->getIdentifier() == CFStringRefII;
   }