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 ofourdan@redhat.com Closes: #647 (closed)