Fixes an infinite loop in readelf parsing a corrupt binary, and other minor corrections.

Fixes an infinite loop in readelf parsing a corrupt binary, and other minor corrections.

	PR binutils/17531
	* readelf.c (get_data): Move excessive length check to earlier on
	in the function and allow for wraparound in the arithmetic.
	(get_32bit_elf_symbols): Terminate early if the section size is
	zero.  Check for an invalid sh_entsize.  Check for an index
	section with an invalid size.
	(get_64bit_elf_symbols): Likewise.
	(process_section_groups): Check for an invalid sh_entsize.