handle labels with 8-bit byte values in dn_skipname

The original logic considered each byte until it either found a 0
value or a value >= 192. This means if a string segment contained any
byte >= 192 it was interepretted as a compressed segment marker even
if it wasn't in a position where it should be interpretted as such.

The fix is to adjust dn_skipname to increment by each segments size
rather than look at each character. This avoids misinterpretting
string segment characters by not considering those bytes.

src/network/dn_skipname.c

@@ -2,11 +2,14 @@
 
 int dn_skipname(const unsigned char *s, const unsigned char *end)
 {
-	const unsigned char *p;
-	for (p=s; p<end; p++)
+	const unsigned char *p = s;
+	while (p < end)
 		if (!*p) return p-s+1;
 		else if (*p>=192)
 			if (p+1<end) return p-s+2;
 			else break;
+		else
+			if (end-p<*p+1) break;
+			else p += *p + 1;
 	return -1;
 }