[libbacktrace] Avoid realloc with size == 0 in backtrace_vector_release

As of C17, realloc with size 0 is marked as an obsolescent feature.

Fix this in backtrace_vector_release by using free instead.

Bootstrapped and reg-tested on x86_64.

2018-11-27  Tom de Vries  <tdevries@suse.de>

	* alloc.c (backtrace_vector_release): Handle vec->size == 0 using free
	instead of realloc.
	* Makefile.am (check_PROGRAMS): Add unittest.
	* Makefile.in: Regenerate.
	* unittest.c: New file.

From-SVN: r266504