Bug 1459701 - Don't crash if ContentParent::UnregisterRemoteFrame is called with a ContentParentId that doesn't exist; r=jimm

MozReview-Commit-ID: AUnuR6W2S5S

--HG--
extra : rebase_source : 3b96e03acb419169782ee0436adb225900a5a182
extra : source : d95af3da7cc6f39f0404622b2293c55dc13265c8

dom/ipc/ContentParent.cpp

@@ -4408,6 +4408,10 @@ ContentParent::UnregisterRemoteFrame(const TabId& aTabId,
     ContentProcessManager* cpm = ContentProcessManager::GetSingleton();
     ContentParent* cp = cpm->GetContentProcessById(aCpId);
 
+    if (!cp) {
+      return;
+    }
+
     cp->NotifyTabDestroyed(aTabId, aMarkedDestroying);
 
     ContentProcessManager::GetSingleton()->UnregisterRemoteFrame(aCpId, aTabId);