glsl: initialize glsl_struct_field properly

don't rely on ralloc doing memset

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Tested-by: Edmondo Tommasina <edmondo.tommasina@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Kenneth Graunke <kenneth@whiteacpe.org>
2 files changed