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
3 jobs for issue647 in 11 minutes and 2 seconds (queued for 27 minutes and 33 seconds)
Status Job ID Name Coverage
  Docker Image
passed #165420
debian-testing

00:00:57

 
  Build And Test
passed #165421
autotools-build-and-test

00:08:15

passed #165422
meson-build-and-test

00:01:48