It is possible for bpf to return a length such that:

It is possible for bpf to return a length such that:

	length != BPF_WORDALIGN(length)

This meeans that it is possible for this to be true:

	interface->rbuf_offset > interface->rbuf_len

Handle this case in the test for running out of packets.  While
OpenBSD's solution of setting interface->rbuf_len to
BPF_WORDALIGN(length) is safe due to the size of the buffer, I think
this solution results in less hidden assumptions.

This should fix the problem of dhclient running away and consuming 100%
CPU.

PR:		bin/102226
Submitted by:	Joost Bekkers <joost at jodocus.org>
MFC after:	3 days