hyperv/vmbus: Make sure that the sub-channel count is valid.

MFC after:	1 week
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D7865

sys/dev/hyperv/vmbus/vmbus_chan.c

@@ -1346,6 +1346,8 @@ vmbus_subchan_get(struct vmbus_channel *pri_chan, int subchan_cnt)
 	struct vmbus_channel **ret, *chan;
 	int i;
 
+	KASSERT(subchan_cnt > 0, ("invalid sub-channel count %d", subchan_cnt));
+
 	ret = malloc(subchan_cnt * sizeof(struct vmbus_channel *), M_TEMP,
 	    M_WAITOK);