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.
Add missing commas to .Xr.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Fix mdoc warning by removing empty lines.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Merge remote-tracking branch 'freebsd/master' into hardened/current/master