Fix corner case where m_len was not being initialized.

Submitted by:	Maksim Yevmenkin <myevmenk@digisle.net>
MFC after:	1 week

sys/kern/uipc_mbuf.c

@@ -588,8 +588,10 @@ m_split(struct mbuf *m0, int len0, int wait)
 			if (n->m_next == NULL) {
 				(void) m_free(n);
 				return (NULL);
-			} else
+			} else {
+				n->m_len = 0;
 				return (n);
+			}
 		} else
 			MH_ALIGN(n, remain);
 	} else if (remain == 0) {