btrfs-progs: check: prevent attempt to insert extent record with max_size==0

btrfs-progs: check: prevent attempt to insert extent record with max_size==0

When this happens, we will trip a BUG_ON(end < start) in insert_state
because in check_extent_refs, we use this max_size expecting it's not zero:

  set_extent_dirty(root->fs_info->excluded_extents,
                   rec->start,
                   rec->start + rec->max_size - 1);

See https://bugzilla.redhat.com/show_bug.cgi?id=1435567
for an example where this scenario occurs.

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.com>