glx: fix crash with bad fbconfig

glx: fix crash with bad fbconfig

GLX documentation states:
	glXCreateNewContext can generate the following errors: (...)
	GLXBadFBConfig if config is not a valid GLXFBConfig

Function checks if the given config is a valid config and sets proper
error code.

Fixes currently crashing glx-fbconfig-bad Piglit test.

v2: coding style cleanups (Emil, Topi)
    use DefaultScreen macro (Emil)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: "11.2" <mesa-stable@lists.freedesktop.org>