tiling: fix zero sized zone
tiling: fix zero sized zone

when plugging a screen in and out, there is the case that a zone has a
usefull geometry of 0x0, which means all clients on this zone are
resized to 0x0. Which leads to a CRIT message in the compositor, which
leads (ref commit 5d875e6a3dcd6a045610a6333cad9b6d15ebb2f1) to a abort()
which is really really annoying. Give here a short ERR about that case
so we are leaving out the compositor, since this really only strikes on
tiling, since in normal mode the client just keeps its size.

tasn/enlightenment,tasn/enlightenment,tasn/enlightenment