mesa: Fix up program_parse.y to avoid uninitialized $$

mesa: Fix up program_parse.y to avoid uninitialized $$

Without this patch, $$.negate, $$.rgba_valid, and $$.xyzw_valid take
on garbage values.  At the moment this problem is benign (the garbage
values happen to be zero), but in my experiments executing GL
operations on a background thread, the garbage values change, leading
to piglit failures.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>