PR21990, Integer overflow in process_version_sections

PR21990, Integer overflow in process_version_sections

This tidies some of the overflow checking when processing verneed
and verdef sections.

	PR 21990
	* readelf.c (process_version_sections <SHT_GNU_verneed>): Check
	for invalid vn_next field before adding to idx.  Use unsigned
	long for index vars.  Move index checks.
	<SHT_GNU_verdef>: Likewise for vd_next.