block: Fix regression for MinGW (assertion caused by short string)

block: Fix regression for MinGW (assertion caused by short string)

The local string tmp_filename is passed to function get_tmp_filename
which expects a string with minimum size MAX_PATH for w32 hosts.

MAX_PATH is 260 and PATH_MAX is 259, so tmp_filename was too short.

Commit eba25057b9a5e19d10ace2bc7716667a31297169 introduced this
regression.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
(cherry picked from commit 89c9bc3d147fdaa932db99b0463b4af1d3e7cda1)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>