http2: avoid null pointer deref in alert output

Bug #4120.

src/output-json-alert.c

@@ -167,6 +167,7 @@ static void AlertJsonHttp2(const Flow *f, const uint64_t tx_id, JsonBuilder *js)
     void *h2_state = FlowGetAppState(f);
     if (h2_state) {
         void *tx_ptr = rs_http2_state_get_tx(h2_state, tx_id);
+        if (tx_ptr) {
             JsonBuilderMark mark = { 0, 0, 0 };
             jb_get_mark(js, &mark);
             jb_open_object(js, "http");
@@ -176,6 +177,7 @@ static void AlertJsonHttp2(const Flow *f, const uint64_t tx_id, JsonBuilder *js)
                 jb_restore_mark(js, &mark);
             }
         }
+    }
 
     return;
 }