st/mesa: initialize members of glsl_to_tgsi_instruction in emit_asm()

st/mesa: initialize members of glsl_to_tgsi_instruction in emit_asm()

This fixes random crashes with MSVC release builds.  It seems the
members are implicitly initialized to zero with gcc, but not MSVC.
In particular, the tex_offset_num_offset field was non-zero causing
a loop over the NULL tex_offsets array to crash.

Zero-init those fields and a few others to be safe.

The regression began with acc23b04cfd64e "ralloc: remove memset from
ralloc_size".

Reviewed-by: Marek Olšák <marek.olsak@amd.com>