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
