json-file: check pointer before using

A check was missing to see if ht_ud was not null before using the
pointer. This should be rare, but it can happen.

Reported-by: Will Metcalf

src/output-json-file.c

@@ -81,7 +81,7 @@ static json_t *LogFileMetaGetUri(const Packet *p, const File *ff) {
         htp_tx_t *tx = AppLayerParserGetTx(IPPROTO_TCP, ALPROTO_HTTP, htp_state, ff->txid);
         if (tx != NULL) {
             HtpTxUserData *tx_ud = htp_tx_get_user_data(tx);
-            if (tx_ud->request_uri_normalized != NULL) {
+            if (tx_ud != NULL && tx_ud->request_uri_normalized != NULL) {
                 char *s = bstr_util_strdup_to_c(tx_ud->request_uri_normalized);
                 if (s != NULL) {
                     js = json_string(s);