If the "pad" field isn't set, Valgrind will report it as uninitialized
memory accesses when the struct is copied into the Display's send buffer.

In practice, this is (probably) harmless, but Valgrind is correct in
believing it's a bug.

https://bugs.freedesktop.org/attachment.cgi?id=133189Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>