arp: without -H <type>, assume "ether". closes 4564

diff --git a/networking/arp.c b/networking/arp.c
index 620f7c0..e2c5bbb 100644
--- a/networking/arp.c
+++ b/networking/arp.c
@@ -443,8 +443,8 @@
 int arp_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int arp_main(int argc UNUSED_PARAM, char **argv)
 {
-	char *hw_type;
-	char *protocol;
+	const char *hw_type = "ether";
+	const char *protocol;
 
 	/* Initialize variables... */
 	ap = get_aftype(DFLT_AF);