slirp: remove dead increments, spotted by clang

Value stored is never read.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/slirp/cksum.c b/slirp/cksum.c
index 34977ff..a044ec1 100644
--- a/slirp/cksum.c
+++ b/slirp/cksum.c
@@ -68,7 +68,9 @@
 
 	if (len < mlen)
 	   mlen = len;
+#ifdef DEBUG
 	len -= mlen;
+#endif
 	/*
 	 * Force to even boundary.
 	 */