ipc: zero initialize message buffer

Fixes valgrind warning

==338826== Thread 42:
==338826== Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s)
==338826==    at 0xC0A146D: sendmsg (in /usr/lib/libc.so.6)
==338826==    by 0x18526D: ipc_send (src/xrt/ipc/shared/ipc_utils.c:65)
==338826==    by 0x157771: ipc_dispatch (ipc_server_generated.c:165)
==338826==    by 0x15C87F: client_loop (src/xrt/ipc/server/ipc_server_per_client_thread.c:112)
==338826==    by 0x15C60C: ipc_server_client_thread (src/xrt/ipc/server/ipc_server_per_client_thread.c:183)
==338826==    by 0xC01A5C1: start_thread (in /usr/lib/libc.so.6)
==338826==    by 0xC09F583: clone (in /usr/lib/libc.so.6)
==338826==  Address 0xc628eafc is on thread 42's stack
==338826==  in frame #2, created by ipc_dispatch (ipc_server_generated.c:23)