lib-imap: Make sure str_unescape() won't be writing past allocated memory

The previous commit should already prevent this, but this makes sure it
can't become broken in the future either. It makes the performance a tiny
bit worse, but that's not practically noticeable.

Author: sirainen

src/lib-imap/imap-parser.c

@@ -267,10 +267,8 @@ static void imap_parser_save_arg(struct imap_parser *parser,
 
 		/* remove the escapes */
 		if (parser->str_first_escape >= 0 &&
-		    (parser->flags & IMAP_PARSE_FLAG_NO_UNESCAPE) == 0) {
-			/* -1 because we skipped the '"' prefix */
-			(void)str_unescape(str + parser->str_first_escape-1);
-		}
+		    (parser->flags & IMAP_PARSE_FLAG_NO_UNESCAPE) == 0)
+			(void)str_unescape(str);
 		arg->_data.str = str;
 		arg->str_len = strlen(str);
 		break;