If the EGLDisplay input parameter to eglInitialize isn't
PRIMARY_DISPLAY or HEADLESS_DISPLAY, egl::Display::get()
will return null. Instead of crashing, we should return
an EGL_BAD_DISPLAY error.

Change-Id: Ie1e39f2807db5a38f8d21fd6e22270c02cbeba3a
Reviewed-on: https://swiftshader-review.googlesource.com/9813Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>