bfd/elf-attrs.c: Fix possible infinite loop parsing attributes

bfd/elf-attrs.c: Fix possible infinite loop parsing attributes

Handle the case of a zero length section or sub-section in
_bfd_elf_parse_attributes and in doing so prevent an infinite loop
in the parser.

bfd/ChangeLog:

2014-11-06  Will Newton  <will.newton@linaro.org>

	* elf-attrs.c (_bfd_elf_parse_attributes): Handle zero
	length sections and sub-sections.