Return unique_xmalloc_ptr from gdbscm_safe_eval_string

Return unique_xmalloc_ptr from gdbscm_safe_eval_string

This changes gdbscm_safe_eval_string to return a unique_xmalloc_ptr.
This allows for the removal of some cleanups.  It also fixes a
potential latent memory leak in gdbscm_set_backtrace.

gdb/ChangeLog
2018-07-17  Tom Tromey  <tom@tromey.com>

	* guile/guile.c (gdbscm_eval_from_control_command): Update.
	* guile/guile-internal.h (gdbscm_safe_eval_string): Update.
	* guile/scm-objfile.c (gdbscm_execute_objfile_script): Update.
	* guile/scm-safe-call.c (gdbscm_safe_eval_string): Return
	unique_xmalloc_ptr.