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: xorg/xserver#647
(Cherry picked from commit 74479a99)
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