vmxnet3: skip zero-length descriptor in the middle of a packet

Passing up such descriptors to iflib is obviously wasteful.
But the main conern is that we may overrun iri_frags array because of
them. That's been observed in practice.

Also, assert that the number of fragments / descriptors / segments is
less than IFLIB_MAX_RX_SEGS.

Reviewed by: gallatin, pkelsey
MFC after: 3 weeks
Sponsored by: Panzura LLC
Differential Revision: https://reviews.freebsd.org/D33189