[libbacktrace] Fix memory leak in loop in build_address_map

When failing in build_address_map, we free the unit that's currently being
handled in the loop, but the ones that already have been allocated are leaked.

Fix this by keeping track of allocated units in a vector, and releasing them
upon failure.

Also, now that we have a vector of allocated units, move the freeing upon
failure of the abbrevs associated with each unit to build_address_map, and
remove the now redundant call to free_unit_addrs_vector.

Bootstrapped and reg-tested on x86_64.

2018-12-28  Ian Lance Taylor  <iant@golang.org>
	    Tom de Vries  <tdevries@suse.de>

	PR libbacktrace/88063
	* dwarf.c (free_unit_addrs_vector): Remove.
	(build_address_map): Keep track of allocated units in vector.  Free
	allocated units and corresponding abbrevs upon failure.  Remove now
	redundant call to free_unit_addrs_vector.  Free addrs vector upon
	failure.  Free allocated unit vector.

Co-Authored-By: Tom de Vries <tdevries@suse.de>

From-SVN: r267443