[libbacktrace] Reduce memory usage in build_address_map

In build_address_map we allocate a unit, and then look for addresses in the
unit, which we store in the addrs vector, with the elements pointing to the
unit.  However, if we cannot find addresses in the unit, the allocated unit is
not used.

Fix this by detecting if the allocated unit has been used, and reusing it
otherwise.

Bootstrapped and reg-tested on x86_64.

2018-12-28  Tom de Vries  <tdevries@suse.de>

	* dwarf.c (build_address_map): Reuse unused units.

From-SVN: r267445