migration/block-dirty-bitmap: fix memory leak in dirty_bitmap_load_bits

migration/block-dirty-bitmap: fix memory leak in dirty_bitmap_load_bits

Release buf on error path too.

Bug was introduced in b35ebdf076d697bc "migration: add postcopy
migration of dirty bitmaps" with the whole function.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180427142002.21930-3-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 16a2227893dc1d5cad78ed376ad1d7e300978fbe)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>