Mini-OS: fix 9pfs frontend error path

Mini-OS: fix 9pfs frontend error path

The early error exit in p9_stat() returns without zeroing the p9_stat
buffer, resulting in free() being called with an uninitialized pointer.

Fix that by calling free_stat() in p9_stat() in case of returning an
error and potentially having allocated strings.

Reported-by: Julien Grall <julien@xen.org>
Fixes: 2d1dfccd3aa3 ("Mini-OS: add read and write support to 9pfsfront")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>