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### Description

<!-- Explain what you did, what you expected to happen, and what actually happened. --> When registering a soft client running on my Android phone (Grandstream Wave), Kamailio crashes every time when the client registers. When setting the debug level below 3 Kamailio does not crash. I have only found the problem with this SIP client. Others are working fine.

### Troubleshooting

#### Reproduction

<!-- If the issue can be reproduced, describe how it can be done. --> Set debug level greather than 2 and register the Grandstream Wave soft client from an Android phone. Kamailio then crashes.

#### Log Messages

<!-- Check the syslog file and if there are relevant log messages printed by Kamailio, add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site). -->

just before crashing I get the following message: 0(7819) ERROR: <core> [core/udp_server.c:484]: udp_rcv_loop(): print buffer building failed (10/16/4)

### Possible Solutions

<!-- If you found a solution or workaround for the issue, describe it. Ideally, provide a pull request with a fix. --> The problem seems to come from non-printable characters sent by the SIP client causing Kamailio to crash.

Changing line 482 in udp_server.c from l = snprintf(printbuf+j, 6, " %02X ", buf[i]); to l = snprintf(printbuf+j, 6, " %02X ", (unsigned char)buf[i]);

fixes the problem.

The compiler treats buf[i] as a signed integer and prepends 'FFFFFF' to the string making it longer than 6 characters. Casting buf[i] as an unsigned char prevents Kamailio from crashing.

### Additional Information

* **Kamailio Version** - output of `kamailio -v`

version: kamailio 5.1.0 (x86_64/linux) flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144, MAX_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. id: unknown compiled with gcc 5.3.1

* **Operating System**:

<!-- Details about the operating system, the type: Linux (e.g.,: Debian 8.4, Ubuntu 16.04, CentOS 7.1, ...), MacOS, xBSD, Solaris, ...; Kernel details (output of `uname -a`) -->

Ubuntu 16.04 (Xenial)