Zero-initialize struct msghdr

==27395== Syscall param socketcall.sendmsg(msg.msg_iov[i]) points to uninitialised byte(s)
==27395==    at 0x546C6A0: __sendmsg_nocancel (syscall-template.S:82)
==27395==    by 0x4E30880: lttcomm_send_creds_unix_sock (sessiond-comm.c:449)
==27395==    by 0x4E2F584: ask_sessiond (lttng-ctl.c:99)
==27395==    by 0x4E2FC85: lttng_list_tracepoints (lttng-ctl.c:667)
==27395==    by 0x4039C1: cmd_list (list.c:314)
==27395==    by 0x40217C: main (lttng.c:266)

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>