s3:smbd: change FNUM_FIELD_INVALID from -1 to 0

s3:smbd: change FNUM_FIELD_INVALID from -1 to 0

This is in preparation for changing the type of files_struct.fnum
to unit64_t.

This can safely be done, since all checks have been converted to
using FNUM_FIELD_INVALID and fsp->fnum is only ever set to either
FNUM_FIELD_INVALID or some i + FILE_HANDLE_OFFSET with i >= 0 and
FILE_HANDLE_OFFSET > 0.

Signed-off-by: Stefan Metzmacher <metze@samba.org>