msg_socket: fix MsgOnSocket::msg_size for Option<T>

This caused None values to be transmitted as zero-bytes, which usually
caused not enough buffer space to be allocated for sending.

BUG=None
TEST=cargo test -p msg_socket

Change-Id: If12d2df56689a84054405f2ba15d1c268afad15e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2197776
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Zach Reizner <zachr@chromium.org>
Commit-Queue: Zach Reizner <zachr@chromium.org>

msg_socket/src/msg_on_socket.rs

@@ -175,7 +175,7 @@ impl<T: MsgOnSocket> MsgOnSocket for Option<T> {
     fn msg_size(&self) -> usize {
         match self {
             Some(v) => v.msg_size() + 1,
-            None => 0,
+            None => 1,
         }
     }