fix invalid free() in error path

the tuid isn't actually allocated - it's a pointer into the raw data.

amends a5a8783e.

src/drv_imap.c

@@ -1186,7 +1186,6 @@ parse_fetch_rsp( imap_store_t *ctx, list_t *list, char *s ATTR_UNUSED )
 	return LIST_OK;
 
   ffail:
-	free( tuid );
 	free( msgid );
 	return LIST_BAD;
 }