btrfs: fix error pointer dereference after failure to allocate fs devices

At device_list_add() we allocate a btrfs_fs_devices structure and then
before checking if the allocation failed (pointer is ERR_PTR(-ENOMEM)),
we dereference the error pointer in a memcpy() argument if the feature
BTRFS_FEATURE_INCOMPAT_METADATA_UUID is enabled.
Fix this by checking for an allocation error before trying the memcpy().

Fixes: f7361d8c

 ("btrfs: sipmlify uuid parameters of alloc_fs_devices()")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>