output-json-alert: don't decref used object

In the unlikely case of a allocation error we will still use the
existing object so it should not be decref and freed.

src/output-json-alert.c

@@ -246,7 +246,6 @@ void AlertJsonHeader(const Packet *p, const PacketAlert *pa, json_t *js)
 
     json_t *ajs = json_object();
     if (ajs == NULL) {
-        json_decref(js);
         return;
     }