[libbacktrace] Don't point to released memory in backtrace_vector_release

When backtrace_vector_release is called with vec.size == 0, it releases the
memory pointed at by vec.base.

Set vec.base set to NULL if vec.size == 0 to ensure we don't point to released
memory.

Bootstrapped and reg-tested on x86_64.

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

	* mmap.c (backtrace_vector_release): Same.
	* unittest.c (test1): Add check.

From-SVN: r266505