tidy elf attr error handling

tidy elf attr error handling

Add a missing error check, and make another error check a little more
stringent.  If it were ever possible for oav2_parse_attr to return
zero, the loop would not terminate.

	* elf-attrs.c (oav2_parse_subsection): Check read_ntbs return
	for errors.  Tidy loop reading attrs, and error on <= 0.