glsl: fix crashes on out of bound matrix access using constant index

glsl: fix crashes on out of bound matrix access using constant index

Fixes these piglit tests:
- spec@glsl-1.20@execution@matrix-out-of-bounds-access@fs-mat4-out-of-bounds-2
- spec@glsl-1.20@execution@matrix-out-of-bounds-access@fs-mat4-out-of-bounds-4
- spec@glsl-1.20@execution@matrix-out-of-bounds-access@fs-mat4-out-of-bounds-5

Signed-off-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Reviewed-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4998>