libbacktrace: avoid ambiguous binary search

Searching for a range match can cause the search order to not match
the sort order, which can cause libbacktrace to miss matching entries.
Allocate an extra entry at the end of function_addrs and unit_addrs vectors,
so that we can safely compare to the next entry when searching.
Adjust the matching code accordingly.

Fixes https://github.com/ianlancetaylor/libbacktrace/issues/44.

	* dwarf.c (function_addrs_search): Compare against the next entry
	low address, not the high address.
	(unit_addrs_search): Likewise.
	(build_address_map): Add a trailing unit_addrs.
	(read_function_entry): Add a trailing function_addrs.
	(read_function_info): Likewise.
	(report_inlined_functions): Search backward for function_addrs
	match.
	(dwarf_lookup_pc): Search backward for unit_addrs and
	function_addrs matches.