Factor out new function backtrace_vector_free.

Bootstrapped and reg-tested on x86_64.

2018-11-30  Tom de Vries  <tdevries@suse.de>

	* internal.h (backtrace_vector_free): New static inline fuction,
	factored out of ...
	* dwarf.c (read_line_info): ... here.

From-SVN: r266658