glamor: pixmap FBO may not be allocated

If `_glamor_create_tex()` fails to allocate the FBO because of
GL_OUT_OF_MEMORY error, the `pixmap_priv->fbo` is NULL.

However, `glamor_get_pixmap_texture()` doesn't actually check whether
the `pixmap_priv->fbo` is NULL and will segfault with a NULL pointer
dereference trying to access the `pixmap_priv->fbo->tex`.

Signed-off-by: Olivier Fourdan <>
Closes: xorg/xserver#647
(Cherry picked from commit 74479a99)
3 jobs for server-1.20-branch in 6 minutes and 24 seconds (queued for 2 seconds)