PR29230, segv in lookup_symbol_in_variable_table

PR29230, segv in lookup_symbol_in_variable_table

The PR23230 testcase uses indexed strings without specifying
SW_AT_str_offsets_base.  In this case we left u.str with garbage (from
u.val) which then led to a segfault when attempting to access the
string.  Fix that by clearing u.str.  The patch also adds missing
sanity checks in the recently committed read_indexed_address and
read_indexed_string functions.

	PR 29230
	* dwarf2.c (read_indexed_address): Return uint64_t.  Sanity check idx.
	(read_indexed_string): Use uint64_t for str_offset.  Sanity check idx.
	(read_attribute_value): Clear u.str for indexed string forms when
	DW_AT_str_offsets_base is not yet read or missing.