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>

src/compiler/nir/nir_print.c

@@ -257,7 +257,7 @@ static const char *
 get_var_name(nir_variable *var, print_state *state)
 {
    if (state->ht == NULL)
-      return var->name;
+      return var->name ? var->name : "unnamed";
 
    assert(state->syms);