We can fail the blk_insert_bs() at init_blk_migration(), leaving the
BlkMigDevState without a dirty_bitmap and BlockDriverState. Account
for the possibly missing elements when doing cleanup.

Fix the following crashes:

Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
0x0000555555ec83ef in bdrv_release_dirty_bitmap (bitmap=0x0) at ../block/dirty-bitmap.c:359
359         BlockDriverState *bs = bitmap->bs;
 #0  0x0000555555ec83ef in bdrv_release_dirty_bitmap (bitmap=0x0) at ../block/dirty-bitmap.c:359
 #1  0x0000555555bba331 in unset_dirty_tracking () at ../migration/block.c:371
 #2  0x0000555555bbad98 in block_migration_cleanup_bmds () at ../migration/block.c:681

Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
0x0000555555e971ff in bdrv_op_unblock (bs=0x0, op=BLOCK_OP_TYPE_BACKUP_SOURCE, reason=0x0) at ../block.c:7073
7073        QLIST_FOREACH_SAFE(blocker, &bs->op_blockers[op], list, next) {
 #0  0x0000555555e971ff in bdrv_op_unblock (bs=0x0, op=BLOCK_OP_TYPE_BACKUP_SOURCE, reason=0x0) at ../block.c:7073
 #1  0x0000555555e9734a in bdrv_op_unblock_all (bs=0x0, reason=0x0) at ../block.c:7095
 #2  0x0000555555bbae13 in block_migration_cleanup_bmds () at ../migration/block.c:690

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Message-id: 20230731203338.27581-1-farosas@suse.de
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>