I've created a surfaceless OpenGL context using the EGL_KHR_surfaceless_context
extension together with eglGetPlatformDisplay/EGL_PLATFORM_GBM_MESA.
In this scenario there seem to be no valid default framebuffer which is Ok, as
I'm normally only render to FBOs with textures bound. Due to an error on my
side I called glDrawBuffer with GL_FRONT_LEFT while no FBO was bound.

This result in a crash in intel_buffers.c because in intelDrawBuffer()
dri2InvalidateDrawable is called with a null pointer which is
not checked in dri2InvalidateDrawable() or anywhere before.

While the root cause for triggering the error is on my side,
I think it may be better to raise an error instead of crashing.
So I propose to add a check to brw->driContext->driDrawablePriv
within intelDrawBuffer. Probably if the driDrawablePriv is nullptr
one should not call intel_prepare_render either. 

Mesa 17.1 with Kernel 4.9.6 on Intel Apollo Lake GPU