asan: more readelf leaks

	* elfcomm.c (get_archive_member_name): Always return malloc'd
	string or NULL.
	* elfedit.c (process_archive): Tidy memory on all return paths.
	* readelf.c (process_archive): Likewise.
	(process_symbol_table): Likewise.
	(ba_cache): New, replacing ..
	(get_symbol_for_build_attribute): ..static vars here.  Free
	strtab and symtab before loading new ones.  Reject symtab without
	valid strtab in loop, breaking out of loop on valid symtab.
	(process_file): Free ba_cache symtab and strtab here, resetting
	ba_cache.