egl-x11: Fix a crash when the app has non-Cogl windows
egl-x11: Fix a crash when the app has non-Cogl windows

The handler for ConfigureNotify events in the EGL X11 winsys was
incorrectly trying dereference the onscreen pointer even if it didn't
find an onscreen for the X window that has resized. This meant that if
the application has other windows that weren't created by Cogl then it
would crash when handling events for them.

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>

djdeath/cogl,gcampax/cogl,djdeath/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl,gcampax/cogl