core: reset cur_opt shortcut after use, it points to local variable

- safety to prevent invalid access on future extensions

(cherry picked from commit 1faf19fa7d34528b6cc15ffcb1ec91a6c6074e90)
(cherry picked from commit eb3f7cbc27fd678891910a14377ef0c5a3477bb0)

cfg_parser.c

@@ -732,7 +732,8 @@ int sr_cfg_parse(cfg_parser_t* st)
 			}
 
 			st->cur_opt = &t;
-			if (process_option(st, opt) < 0) return -1;
+			if (process_option(st, opt) < 0) { st->cur_opt = 0; return -1; }
+			st->cur_opt = 0;
 			break;
 
 		case '[':