bfd_get_file_size calls

bfd_get_file_size can return 0, meaning the file size is unknown.

	* coffgen.c (_bfd_coff_get_external_symbols): Don't call
	bfd_get_file_size twice.
	(_bfd_coff_read_string_table): Allow for bfd_get_file_size
	zero, ie. unknown, return.
	* elf-attrs.c (_bfd_elf_parse_attributes): Likewise.
	* elfcode.h (elf_swap_shdr_in): Likewise.
	(elf_object_p): Don't call bfd_get_file_size twice and correct
	file size check.