[RFA 6/8] Remove cleanup from backtrace_command

[Yao Qi]

Tom Tromey <tom@tromey.com> writes:

> gdb/ChangeLog
> 2017-10-13  Tom Tromey  <tom@tromey.com>
>
> 	* stack.c (backtrace_command): Use std::string.
> 	(backtrace_command_1): Make "count_exp" const.

Patch is good to me.

-- 
Yao (齐尧)