[libbacktrace] Fix segfault upon allocation failure

If the allocation of abbrevs->abbrevs in read_abbrevs fails, then
abbrevs->num_abbrevs remains nonzero, and consequently free_abbrevs will
segfault when accessing abbrevs->abbrevs.

Fix this by setting abbrevs->num_abbrevs only after abbrevs->abbrevs
allocation has succeeded.

Bootstrapped and reg-tested on x86_64.

2018-11-28  Tom de Vries  <tdevries@suse.de>

	* dwarf.c (read_abbrevs): Fix handling of abbrevs->abbrevs allocation
	failure.

From-SVN: r266562