Bug 783134: Work around crashing on startup with fullscreen widgets. r=roc

xpfe/appshell/src/nsWebShellWindow.cpp

@@ -252,7 +252,9 @@ nsWebShellWindow::WindowResized(nsIWidget* aWidget, PRInt32 aWidth, PRInt32 aHei
   }
 
   nsCOMPtr<nsIBaseWindow> shellAsWin(do_QueryInterface(mDocShell));
+  if (shellAsWin) {
     shellAsWin->SetPositionAndSize(0, 0, aWidth, aHeight, false);
+  }
   // Persist size, but not immediately, in case this OS is firing
   // repeated size events as the user drags the sizing handle
   if (!IsLocked())