libsmb: Avoid undefined behaviour when parsing whoami state

libsmb: Avoid undefined behaviour when parsing whoami state

If num_gids is such that the gids array would overflow the rdata buffer,
'p + 8' could produce a result pointing outside the buffer, and thus
result in undefined behaviour. To avoid this, we check num_gids against
the size of the buffer beforehand.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>