st/mesa: Handle st_framebuffer_create returning NULL

st/mesa: Handle st_framebuffer_create returning NULL

st_framebuffer_create returns NULL if stfbi == NULL or
st_framebuffer_add_renderbuffer returns false for the colour buffer.

Fixes Xorg crashing on startup using glamor on radeonsi.

Fixes: 147d7fb772a7 ("st/mesa: add a winsys buffers list in st_context")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101775
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Brian Paul <brianp@vmware.com>