PR28173, nds32_elf_howto_table index out of bounds

PR28173, nds32_elf_howto_table index out of bounds

Indexing the howto table was seriously broken by a missing entry, and
use of assertions about user input rather than testing the input.

	PR 28173
	* elf32-nds32.c (nds32_elf_howto_table): Add missing empty howto.
	(bfd_elf32_bfd_reloc_type_table_lookup): Replace assertions with
	range checks.  Return NULL if unsupported reloc type.  Remove
	dead code.  Take an unsigned int param.
	(nds32_info_to_howto_rel): Test for NULL howto or howto name
	return from lookup.  Remove assertion.
	(nds32_info_to_howto): Remove unnecessary ATTRIBUTE_UNUSED.
	Test for NULL howto or howto name return from lookup.