MFH r275941:

MFH r275941:
Adjust the test of a KASSERT to better match the intent.

This assertion was added in r246213 as a guard against corrupted mbufs
arriving from drivers, the key distinguishing factor of said mbufs being
that they had a negative length. Given we're in a while loop specifically
designed to skip over zero-length mbufs, panicking on a zero-length mbuf
seems incorrect.

Suggested by:	rmacklem
MFH go-ahead:	benno
Approved by:	grog (co-mentor)

git-svn-id: svn://svn.freebsd.org/base/stable/10@276327 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f