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) (Cherry picked from commit 74479a99)