alpha_ecoff_get_relocated_section_contents buffer overflow

alpha_ecoff_get_relocated_section_contents buffer overflow

This is aimed at fixing holes in two alpha-ecoff relocation functions
that access section contents without first bounds checking offsets.
I've also rewritten ALPHA_R_OP_STORE handling to support writing to
the bytes near the end of the section.

	* coff-alpha.c (alpha_ecoff_get_relocated_section_contents): Don't
	bother checking ALPHA_R_LITERAL insn.  Range check before reading
	contents for ALPHA_R_GPDISP, and simplify handling.  Rewrite
	ALPHA_R_OP_STORE handling.  Correct error callback args.
	(alpha_relocate_section): Similarly.  Don't abort, report errors.