workspace: Fix an incorrect signal check

workspace: Fix an incorrect signal check

While commit 3094f863 was intended to cancel the ongoing idle hide
timeout before we start a new one, a mistake slipped in there while
rebasing: Obviously we should check if the signal id is NOT 0 here.

This didn't prevent timeouts being started while old ones are still
running and did override `this._idleHideOverlayId`, which caused the old
timeouts to run indefinitely after an overlay actor was destroyed
because we fail early (and don't return TRUE) in `_idleHideOverlay()`.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/735