connection: Clear correct args when clearing fds to -1

commit 52609ddf was intended to
set fds to -1 in the arg list, however it failed to account for
version information at the start of signatures.

Most noticably, this broke mesa's create_prime_buffer by setting
width to -1 instead of the fd, as the width was the argument
following the fd, and the version was one byte long.

This should close https://bugs.kde.org/show_bug.cgi?id=389200



Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>