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
2 files changed