libceph: NULL deref on osdmap_apply_incremental() error path

There are hidden gotos in the ceph_decode_* macros.  We need to set the
"err" variable on these error paths otherwise we end up returning
ERR_PTR(0) which is NULL.  It causes NULL dereferences in the callers.

Fixes: 6f428df4 ("libceph: pg_upmap[_items] infrastructure")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[idryomov@gmail.com: similar bug in osdmap_decode(), changelog tweak]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>