[wayland] Avoid compare to unintialised value in keyboard repeat

Summary:
If we get a key event for which

if (m_xkb->shouldKeyRepeat(key) &&
waylandServer()->seat()->keyRepeatDelay() != 0) fails

m_key will be unitialised and on release we have a compare against
unitialised memory.

Won't do any harm, it'll just stop a timer that isn't running, but
valgrind complains.

0 is the value QKeyEvent uses when nativeScanCode is unknown so a safe
initial values.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23748