Fix crash in "document" command PR cli/29800 points out that "document" will now crash when the argument is an undefined command. This is a regression due to the "document user-defined aliases" patch. Approved-By: Joel Brobecker <brobecker@adacore.com> Reviewed-By: Philippe Waroquiers <philippe.waroquiers@skynet.be> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29800