Fixes a crashy over-releasing bug in FF plugin.

We were failing to AddRef the topWin reference, which would cause random crashes later.

Patch by: me, jat (pair prog)
Review by: jat

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7206 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/plugins/xpcom/ExternalWrapper.cpp b/plugins/xpcom/ExternalWrapper.cpp
index 54bf14a..5ce3367 100644
--- a/plugins/xpcom/ExternalWrapper.cpp
+++ b/plugins/xpcom/ExternalWrapper.cpp
@@ -177,7 +177,7 @@
         << Debug::flush;
     return false;
   }
-  *topWinRet = topWinInt;
+  NS_ADDREF(*topWinRet = topWinInt);
   return true;
 }