* ada-valprint.c (ada_print_scalar): Accept NULL type argument

	* ada-valprint.c (ada_print_scalar): Accept NULL type argument
	to indicate scalar should be printed as default integer.
	(print_optional_low_bound): Pass NULL to ada_print_scalar to
	indicate default integer output.
	* ada-typeprint.c (print_range, print_range_bound): Likewise.
	(print_choices): Likewise.  Thus, accept NULL as val_type.
	* ada-lang.c (ada_variant_discrim_type): Return NULL when failed
	to look up controlling discriminant name.