db_mongodb: re-init iterator for bson covert search failure

src/modules/db_mongodb/mongodb_dbase.c

@@ -529,9 +529,15 @@ static int db_mongodb_convert_bson(const db1_con_t* _h, db1_res_t* _r,
 		LM_DBG("looking for field[%d] named: %s\n", col, colname);
 		if(mgres->colsdoc) {
 			if(!bson_iter_find(&riter, colname)) {
-				LM_ERR("field [%s] not found in result iterator\n",
+				if (!bson_iter_init (&riter, _rdoc)) {
+					LM_ERR("failed to initialize result iterator\n");
+					return -3;
+				}
+				if(!bson_iter_find(&riter, colname)) {
+					LM_ERR("field [%s] not found in result iterator\n",
 						colname);
-				return -4;
+					return -4;
+				}
 			}
 			piter = &riter;
 		} else {