egl: Check for NULL native_window in eglCreateWindowSurface

egl: Check for NULL native_window in eglCreateWindowSurface

We have customers using NULL as a way to test the robustness of the API.
Without this check, EGL will segfault trying to dereference
dri2_surf->wl_win->private because wl_win is NULL.

This fix adds a check and sets EGL_BAD_NATIVE_WINDOW

v2: Incorporated feedback from idr - moved the check to a higher level
function.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Cc: "10.1 10.2" <mesa-stable@lists.freedesktop.org>