r222015 introduced a new assertion that the size of a fixed-length sbuf

buffer is greater than 1.  This triggered panics in at least one spot in
the kernel (the MAC Framework) which passes non-negative, rather than >1
buffer sizes based on the size of a user buffer passed into a system
call.  While 0-size buffers aren't particularly useful, they also aren't
strictly incorrect, so loosen the assertion.

Discussed with:	phk (fears I might be EDOOFUS but willing to go along)
Spotted by:	pho + stress2
Approved by:	re (kib)

sys/kern/subr_sbuf.c

@@ -184,7 +184,7 @@ sbuf_newbuf(struct sbuf *s, char *buf, int length, int flags)
 	s->s_buf = buf;
 
 	if ((s->s_flags & SBUF_AUTOEXTEND) == 0) {
-		KASSERT(s->s_size > 1,
+		KASSERT(s->s_size >= 0,
 		    ("attempt to create a too small sbuf"));
 	}