glamor: pixmap FBO may not be allocated (cherry-pick to 1.20)
_glamor_create_tex() fails to allocate the FBO because of
GL_OUT_OF_MEMORY error, the
pixmap_priv->fbo is NULL.
glamor_get_pixmap_texture() doesn't actually check whether
pixmap_priv->fbo is NULL and will segfault with a NULL pointer
dereference trying to access the