st/mesa: Fix NULL pointer dereference for incomplete framebuffers
This can happen with glamor, which uses EGL_KHR_surfaceless_context and only explicitly binds GL_READ_FRAMEBUFFER for glReadPixels. Cc: mesa-stable@lists.freedesktop.org Reviewed-by:Brian Paul <brianp@vmware.com> (cherry picked from commit 136c437c)