Avoid allocating over-large buffers when parsing corrupt binaries.

Avoid allocating over-large buffers when parsing corrupt binaries.

	PR binutils/17512
	* coffgen.c (_bfd_coff_get_external_symbols): Do not try to load a
	symbol table bigger than the file.
	* elf.c (bfd_elf_get_str_section): Do not try to load a string
	table bigger than the file.

	* readelf.c (process_program_headers): Avoid memory exhaustion due
	to corrupt values in a dynamis segment header.
	(get_32bit_elf_symbols): Do not attempt to read an over-large
	section.
	(get_64bit_elf_symbols): Likewise.