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;
 		}

src/linux/device.c

@@ -131,6 +131,7 @@ static bool read_packet(vpn_packet_t *packet) {
 				return false;
 			}
 
+			memset(packet->data, 0, 12);
 			packet->len = inlen + 10;
 			break;
 		case DEVICE_TYPE_TAP:

src/solaris/device.c

@@ -151,6 +151,7 @@ static bool read_packet(vpn_packet_t *packet) {
 			return false;
 	}
 
+	memset(packet->data, 0, 12);
 	packet->len = inlen + 14;
 
 	device_total_in += packet->len;