Fix implementation of readelf's -wE and -wN options,

	* dwarf.c (dwarf_select_sections_by_name): If the entry's value is
	zero then clear the corresponding variable.
	(dwarf_select_sections_by_letters): Likewise.
	* testsuite/binutils-all/debuginfo.exp: Expect -WE and -wE
	debuginfod tests to fail.