Silence an unused-variable warning with gcc opt. No bug, r=themaid

--HG--
extra : rebase_source : e4c0ff6af6a2bbf60de583f1f19c3329ee8e26fc

js/src/vm/Debugger.cpp

@@ -3379,6 +3379,7 @@ DebuggerFrame_setOnPop(JSContext *cx, unsigned argc, Value *vp)
 {
     REQUIRE_ARGC("Debugger.Frame.set onPop", 1);
     THIS_FRAME(cx, argc, vp, "set onPop", args, thisobj, fp);
+    (void) fp;  // Silence GCC warning
     if (!IsValidHook(args[0])) {
         JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, JSMSG_NOT_CALLABLE_OR_UNDEFINED);
         return false;