ndis(4): Avoid overflow.

This is a long standing problem: our random() function returns an
unsigned integer but the rand provided by ndis(4) returns an int.
Scale it down.

MFC after:	2 weeks

sys/compat/ndis/subr_ntoskrnl.c

@@ -3189,7 +3189,7 @@ static int
 rand(void)
 {
 
-	return (random());
+	return (random() / 2 + 1);
 }
 
 static void