When releasing all keys, we send a key release for scancode in the
key_state array, including the very first element with scancode 0.
send_key() complain when the scancode is 0, so make sure we don't call it
for this first element.

This fixes a
(remote-viewer:25548): GSpice-CRITICAL **: send_key: assertion 'scancode != 0' failed
warning when starting remote-viewer windowed, and then switching to another
desktop client-side using ctrl+alt+arrow.