Fixed a crash in md4 routines when using them on files with zero length, reported by Lukasz Saduniowski.

code/qcommon/md4.c

@@ -159,10 +159,10 @@ static void mdfour_update(struct mdfour *md, byte *in, int n)
 {
 	uint32_t M[16];
 
-	if (n == 0) mdfour_tail(in, n);
-
 	m = md;
 
+	if (n == 0) mdfour_tail(in, n);
+
 	while (n >= 64) {
 		copy64(M, in);
 		mdfour64(M);