clients: Fix a crash on FreeBSD

A pid is not necessarily a long on all systems, so casting the value of
NET_WM_PID directly into a GPid may cause a stack overflow.

Fixes: 49c1a33a
Closes: #411


Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
(cherry picked from commit a691e6c50ce2d797183c98bc2e9c72b8bb2b2a5a)