reds-stream: Don't use sendmsg with uninitialized memory
reds-stream: Don't use sendmsg with uninitialized memory

On my 64 bit Fedora 25, CMSG_SPACE() adds 4 bytes of padding after the
file descriptor in the control data. This causes warnings when ran under
valgrind as we set msg_controllen to CMSG_SPACE().

This commit fills the control data to 0 to avoid these warnings.

==30301== Syscall param sendmsg(msg.msg_control) points to uninitialised byte(s)
==30301==    at 0x8127367: sendmsg (sendmsg.c:28)
==30301==    by 0x41880B: reds_stream_send_msgfd (reds-stream.c:295)
==30301==    by 0x40953F: main (test-stream.c:121)
==30301==  Address 0xffefff1b4 is on thread 1's stack
==30301==  in frame #1, created by reds_stream_send_msgfd (reds-stream.c:263)

Signed-off-by: Christophe Fergeau <05b95054a663c388a4e33d95828cb30f0370f8ef@redhat.com>
Acked-by: Pavel Grunt <fbda40b445316123f12a5a6bd7556918ea74f4bc@redhat.com>

fgouget/spice,fgouget/spice,fgouget/spice,fgouget/spice