Reconstruct a CSTP packet when received in multiple chunks

When the openconnect server receives a broken into multiple chunks CSTP packet over the main channel it bails out with:

ocserv[16828]: worker[VPN]: {IP} writing 60 byte(s) to TUN
ocserv[16828]: worker[VPN]: {IP} received 272 byte(s) (TLS)
ocserv[16828]: worker[VPN]: {IP} unexpected CSTP length (have 60, should be 264)
ocserv[16828]: worker[VPN]: {IP} worker-vpn.c:1094: error parsing CSTP data

The issue becomes visible only when using the unix socket and if the TLS proxy software breaks a single TLS packet into multiple writes.