nir: don't segfault when printing variables with no name

nir: don't segfault when printing variables with no name

While normally we give variables whose name field is NULL a temporary
name when called from nir_print_shader(), when we were calling from
nir_print_instr() we never bothered, meaning that we just segfaulted
when trying to print out instructions with such a variable. Since
nir_print_instr() is meant to be called while debugging, we don't need
to bother too much about giving a consistent name, but we don't want to
crash in the middle of debugging.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>