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.

No objection from:	kib


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275941 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f