Clear Ethernet header when reading packets from a tun device.

This fixes a warning from valgrind about uninitialized bytes, which were being
sent to other nodes.

src/bsd/device.c

@@ -238,6 +238,7 @@ static bool read_packet(vpn_packet_t *packet) {
 					return false;
 			}
 
+			memset(packet->data, 0, 12);
 			packet->len = inlen + 14;
 			break;
 
@@ -269,6 +270,7 @@ static bool read_packet(vpn_packet_t *packet) {
 					return false;
 			}
 
+			memset(packet->data, 0, 12);
 			packet->len = inlen + 10;
 			break;
 		}