If Zabbix server or proxy closes connection during TLS handshake then zabbix_sender may receive SIGPIPE signal.
Although there is a signal handler in data sending thread of zabbix_sender the SIGPIPE is not handled.
zabbix_sender returns 0 (success), but server/proxy has accepted no data at all.

This issue was noticed when working on support of LibreSSL and OpenSSL with no PSK.

Steps to reproduce: (Note: see a comment below for much simpler way to reproduce)

1. Compile OpenSSL 1.1.1a with no PSK support.
2. Compile Zabbix server to work with this special OpenSSL. As changes are not yet merged you need to take svn://svn.zabbix.com/branches/dev/ZBX-15552-30 branch which can handle OpenSSL with no PSK support.
3. Run server with this special OpenSSL.
4. Run standard zabbix_sender from a machine where zabbix_sender uses usual OpenSSL 1.1.1a (with PSK support enabled). Use connecting with PSK.
5. Run zabbix_sender multiple times. Results are varying from time to time:

   $ zabbix_sender -z 192.168.1.2 -s "host1" --tls-connect psk --tls-psk-identity=testid --tls-psk-file=/home/andris/zabbix_agentd.psk -k trapper_test -o abc
   Sending failed. Use option -vv for more detailed output.
   $ zabbix_sender -z 192.168.1.2 -s "host1" --tls-connect psk --tls-psk-identity=testid --tls-psk-file=/home/andris/zabbix_agentd.psk -k trapper_test -o abc
   sent: 1; skipped: 0; total: 1
   

Expected:
zabbix_sender always reports error.

Could be OpenSSL-specific. It works as expected if zabbix sender uses GnuTLS 3.6.6. Also works if zabbix_sender sends to proxy which uses LibreSSL 2.8.2 (no PSK support).