st_glsl_to_tgsi: don't read potentially uninitialized buffer variable

Found by -fsanitize=undefined. Note that this should be a harmless issue in
practice because the inst->op check always dominates anyway.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
1 file changed