glsl: fix crash compiling bindless samplers inside unnamed UBOs

The check to see if we were dealing with a buffer block was
too late and only worked for named UBOs.

Fixes: f32b01ca "glsl/linker: remove ubo explicit binding handling"

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1900
(cherry picked from commit 1294f01e)