[alpha.webkit.RetainPtrCtorAdoptChecker] Check nullity before calling IgnoreParenCasts. (#137556)

Author: rniwa

clang/lib/StaticAnalyzer/Checkers/WebKit/RetainPtrCtorAdoptChecker.cpp

@@ -444,9 +444,10 @@ class RetainPtrCtorAdoptChecker
       return false;
     if (!ObjCMsgExpr->isInstanceMessage())
       return false;
-    auto *Receiver = ObjCMsgExpr->getInstanceReceiver()->IgnoreParenCasts();
+    auto *Receiver = ObjCMsgExpr->getInstanceReceiver();
     if (!Receiver)
       return false;
+    Receiver = Receiver->IgnoreParenCasts();
     if (auto *Inner = dyn_cast<ObjCMessageExpr>(Receiver)) {
       if (InnerExpr)
         *InnerExpr = Inner;