Bug 708228 - Fix infinite write barrier recursion in regexp purge (r=cdleary)

js/src/jit-test/tests/basic/bug708228.js

@@ -0,0 +1,5 @@
+// |jit-test| error: TypeError
+gczeal(4);
+var g_rx = /(?:)/;
+(3).replace(g_rx.compile("test", "g"), {});
+

js/src/vm/RegExpObject.cpp

@@ -366,7 +366,7 @@ regexp_finalize(JSContext *cx, JSObject *obj)
 static void
 regexp_trace(JSTracer *trc, JSObject *obj)
 {
-    if (IS_GC_MARKING_TRACER(trc))
+    if (trc->runtime->gcRunning)
         obj->asRegExp()->purge(trc->context);
 }