There should not be zero length mbufs in socket buffers.  The code comes
from r1451, and thus can't be explained.  A patch with explicit panic()
here survived all tests.

Tested by:	pho
Sponsored by:	Nginx, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274494 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f