mesa: Don't crash when destroying contexts created with no visual.

mesa: Don't crash when destroying contexts created with no visual.

dEQP-EGL.functional.create_context.no_config tries to create a context
with no config, then immediately destroys it.  The drawbuffer is never
set up, so we can't dereference it asking if it's double buffered, or
we'll crash on a null pointer dereference.

Just bail early.

Applications using EGL_KHR_no_config_context could hit this.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>