glamor: pixmap FBO may not be allocated (cherry-pick to 1.20)

Merged Karol Herbst requested to merge karolherbst/xserver:server-1.20-branch into server-1.20-branch

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 Closes: #647 (closed) (Cherry picked from commit 74479a99)

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