smpp_mirror: Don't allocate msgb's for unrealistic amounts of memory

If the remote ESME would send us 0xffffffff as length field, don't try
to allocte 4GB of memory, but bail out.

Change-Id: I561f75210811826de06ea1673eca1df24faaa210
Fixes: CID#240738

src/utils/smpp_mirror.c

@@ -246,6 +246,10 @@ static int esme_read_cb(struct osmo_fd *ofd)
 			esme->read_idx += rc;
 		if (esme->read_idx >= sizeof(uint32_t)) {
 			esme->read_len = ntohl(len);
+			if (esme->read_len > 65535) {
+				/* unrealistic */
+				goto dead_socket;
+			}
 			msg = msgb_alloc(esme->read_len, "SMPP Rx");
 			if (!msg)
 				return -ENOMEM;
@@ -283,6 +287,7 @@ dead_socket:
 	osmo_fd_unregister(&esme->wqueue.bfd);
 	close(esme->wqueue.bfd.fd);
 	esme->wqueue.bfd.fd = -1;
+	esme_read_state_reset(esme);
 	exit(2342);
 
 	return 0;