Fix another misuse of BOpenHashTable::Iterator

* This ioctl is used only in the arp command, but still we should keep
it working
* Similar to the problem in elf.cpp, this relied on entry being set to
NULL when trying to iterate after the last element.

src/add-ons/kernel/network/datalink_protocols/arp/arp.cpp

@@ -885,14 +885,12 @@ arp_control(const char *subsystem, uint32 function, void *buffer,
 
 			arp_entry *entry = NULL;
 			uint32 i = 0;
-			while (iterator.HasNext() && i < control.cookie) {
+			for (uint32 i = 0; i <= control.cookie; i++) {
+				if (!iterator.HasNext())
+					return B_ENTRY_NOT_FOUND;
 				entry = iterator.Next();
-				i++;
 			}
 
-			if (entry == NULL)
-				return B_ENTRY_NOT_FOUND;
-
 			control.cookie++;
 			control.address = entry->protocol_address;
 			if (entry->hardware_address.sdl_alen == ETHER_ADDRESS_LENGTH) {