Bug 1462885 - Fix iteration in ImageBridgeParent::NotifyImageComposites() r=mattwoodrow

Iteration becomes wrong when ImageBridgeParent::GetInstance() fails in NotifyImageComposites().

Differential Revision: https://phabricator.services.mozilla.com/D18172

--HG--
extra : moz-landing-system : lando

gfx/layers/ipc/ImageBridgeParent.cpp

@@ -337,6 +337,7 @@ class ProcessIdComparator {
     }
     RefPtr<ImageBridgeParent> bridge = GetInstance(pid);
     if (!bridge || bridge->mClosed) {
+      i = end;
       continue;
     }
     bridge->SendPendingAsyncMessages();