secsipid: set global url pointer to null after free

- GH #4284

(cherry picked from commit 15a976808f5e785a6d234734b0a7de0d735d4a5a)

src/modules/secsipid/secsipid_mod.c

@@ -916,6 +916,7 @@ static sr_kemi_xval_t *ki_secsipid_get_url(sip_msg_t *msg, str *surl)
 
 	if(_secsipid_get_url_val.s != NULL) {
 		free(_secsipid_get_url_val.s);
+		_secsipid_get_url_val.s = NULL;
 		_secsipid_get_url_val.len = 0;
 	}
 
@@ -959,6 +960,7 @@ static int w_secsipid_get_url(sip_msg_t *msg, char *purl, char *povar)
 	}
 	if(_secsipid_get_url_val.s != NULL) {
 		free(_secsipid_get_url_val.s);
+		_secsipid_get_url_val.s = NULL;
 		_secsipid_get_url_val.len = 0;
 	}