`bash --debugger` in interactive mode

[Techlive Zheng]

I happened to have `shopt -s extdebug` in my .bashrc recently.

After a new bash session started, I got the following output.

    bash debugger, bashdb, release 4.2-0.8

    Copyright 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011
Rocky Bernstein
    This is free software, covered by the GNU General Public License,
and you are
    welcome to change it and/or distribute copies of it under certain
conditions.

    ** Internal debug error _Dbg_is_file(): file argument null
    bash: _Dbg_filenames[$fullname]: bad array subscript

I guess `--debugger` or `shopt -s extdebug` is not supposed to be run
under interactive mode?

If so, maybe a detection should be done for this situation?