s3: smbd: Ensure brl_get_locks_readonly() always returns a valid struct byte_range_lo...

s3: smbd: Ensure brl_get_locks_readonly() always returns a valid struct byte_range_lock even if there are no locks.

brl_get_locks_readonly() currently returns NULL when it can't
find any byte range locks on the file. This is an error - it
should return a valid struct byte_range_lock containing num_locks == 0
so it can be cached.

Returning NULL when there are no locks causes POSIX lock
tests to fail returning NT_STATUS_NO_MEMORY (as it thinks
it can't allocate the struct) instead of NT_STATUS_OK.

Fixes bug:

Bug 10431 - STATUS_NO_MEMORY response from Query File Posix Lock request

https://bugzilla.samba.org/show_bug.cgi?id=10431

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <Volker.Lendecke@SerNet.DE>