read-cache: use of memory after it is freed

introduced with c46c406ae1 (trace.h: support nested performance tracing)
on Aug 18, 2018 but not affecting maint

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -2038,8 +2038,8 @@ int read_index_from(struct index_state *istate, const char *path,
 	freshen_shared_index(base_path, 0);
 	merge_base_index(istate);
 	post_read_index_from(istate);
-	free(base_path);
 	trace_performance_leave("read cache %s", base_path);
+	free(base_path);
 	return ret;
 }