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

sys/fs/nfs/nfs_commonsubs.c

@@ -219,7 +219,8 @@ nfsm_mbufuio(struct nfsrv_descript *nd, struct uio *uiop, int siz)
 				}
 				mbufcp = NFSMTOD(mp, caddr_t);
 				len = mbuf_len(mp);
-				KASSERT(len > 0, ("len %d", len));
+				KASSERT(len >= 0,
+				    ("len %d, corrupted mbuf?", len));
 			}
 			xfer = (left > len) ? len : left;
 #ifdef notdef