Fix memory corruption in Guile command interface

Fix memory corruption in Guile command interface

Re-registering a command will delete previous commands of the same name,
running the destroyer for the command object.  The Guile destroyer
incorrectly tried to xfree the name and other strings, which is invalid
as they are on the GC heap.

gdb/ChangeLog:

	* guile/scm-cmd.c (cmdscm_destroyer): Don't xfree the name and
	other strings, as these are on the GC'd heap, and will be
	collected along with the smob.