Avoid accessing null pointers when an X11 call fails.

Since EGL doesn't own the X11 window, we expect it to be valid
for the duration of the EGL surface. Fail hard if that's not the case.

Bug chromium:833229
Bug chromium:824522

Change-Id: Iba5e3832fe312fb50232a13e2163a022f5048a76
Reviewed-on: https://swiftshader-review.googlesource.com/19788Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>