PR28403, null pointer dereference in disassemble_bytes

Indexing of symbol and howto arrays wasn't checked in aout targets.

	PR 28403
	* aout-ns32k.c (MY (reloc_howto)): Sanity check howto_table index.
	Make r_index unsigned.
	(MY_swap_std_reloc_in): Make r_index unsigned.
	* aoutx.h (MOVE_ADDRESS): Sanity check symbol r_index.
	(aout_link_input_section_std): Make r_index unsigned.
	(aout_link_input_section_ext): Likewise.
	* i386lynx.c (MOVE_ADDRESS): Sanity check symbol r_index.
	(swap_ext_reloc_in, swap_std_reloc_in): Make r_index unsigned.
	* pdp11.c (MOVE_ADDRESS): Sanity check symbol r_index.