Bug 647600: Restore null checks. r=bsmedberg

modules/plugin/base/src/nsNPAPIPlugin.cpp

@@ -910,13 +910,17 @@ OnShutdown()
 
 AsyncCallbackAutoLock::AsyncCallbackAutoLock()
 {
+  if (sPluginThreadAsyncCallLock) {
     sPluginThreadAsyncCallLock->Lock();
   }
+}
 
 AsyncCallbackAutoLock::~AsyncCallbackAutoLock()
 {
+  if (sPluginThreadAsyncCallLock) {
     sPluginThreadAsyncCallLock->Unlock();
   }
+}
 
 
 NPP NPPStack::sCurrentNPP = nsnull;