libradius: Fix attribute length validation in rad_get_attr(3)

The length of the attribute header needs to be excluded when comparing
the attribute length against the length of the packet. Otherwise,
validation may incorrectly fail when fetching the final attribute in a
message.

Fixes: 8d5c78130 ("libradius: Fix input validation bugs")
Reported by: Peter Eriksson
Tested by: Peter Eriksson
MFC after: now
Sponsored by: The FreeBSD Foundation