Bug 850000 - Don't clobber exceptions set in security wrapper check() hooks. r=mrbkap

js/src/jsproxy.h

@@ -357,7 +357,12 @@ class JS_FRIEND_API(AutoEnterPolicy)
         allow = handler->hasPolicy() ? handler->enter(cx, wrapper, id, act, &rv)
                                      : true;
         recordEnter(cx, wrapper, id);
-        if (!allow && !rv && mayThrow)
+        // We want to throw an exception if all of the following are true:
+        // * The policy disallowed access.
+        // * The policy set rv to false, indicating that we should throw.
+        // * The caller did not instruct us to ignore exceptions.
+        // * The policy did not throw itself.
+        if (!allow && !rv && mayThrow && !JS_IsExceptionPending(cx))
             reportError(cx, id);
     }