Zero out the host's `msg_control` buffer

If this is not done, qemu would drop any control message after the first
one.

This is because glibc's `CMSG_NXTHDR` macro accesses the uninitialized
cmsghdr's length field in order to find out if the message fits into the
`msg_control` buffer, wrongly assuming that it doesn't because the
length field contains garbage. Accessing the length field is fine for
completed messages we receive from the kernel, but is - as far as I know
- not needed since the kernel won't return such an invalid cmsghdr in
the first place.

This is tracked as this glibc bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=13500

It's probably also a good idea to bail with an error if `CMSG_NXTHDR`
returns NULL but `TARGET_CMSG_NXTHDR` doesn't (ie. we still expect
cmsgs).

Signed-off-by: Jonas Schievink <jonasschievink@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20180711221244.31869-1-jonasschievink@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/syscall.c

@@ -3843,6 +3843,8 @@ static abi_long do_sendrecvmsg_locked(int fd, struct target_msghdr *msgp,
     }
     msg.msg_controllen = 2 * tswapal(msgp->msg_controllen);
     msg.msg_control = alloca(msg.msg_controllen);
+    memset(msg.msg_control, 0, msg.msg_controllen);
+
     msg.msg_flags = tswap32(msgp->msg_flags);
 
     count = tswapal(msgp->msg_iovlen);