glsl: Silence a warning at -Og

glsl: Silence a warning at -Og

   ../src/compiler/glsl/gl_nir_link_uniforms.c: In function ‘gl_nir_link_uniforms’:
   ../src/compiler/glsl/gl_nir_link_uniforms.c:1747:39: warning: ‘num_blocks’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    1747 |                for (unsigned i = 0; i < num_blocks; i++) {

Don't have the patience to seee how you'd hit this but doing nothing is
clearly the right edge case behavior.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8724>