sh-mode buglet

[Giorgos Keramidas <keramida@ceid.upatras.gr>]

The head version of sh-script.el has a minor buglet that prevents one
from running:

    % emacs -f sh-mode

to fire up Emacs in sh-mode.

The debugger backtrace is:

    Debugger entered--Lisp error: (wrong-type-argument stringp nil)
      string-match("[.]sh\\>" nil)
      sh-mode()
      eval((sh-mode))
      eval-last-sexp-1((4))
      eval-last-sexp((4))
      call-interactively(eval-last-sexp)

and it looks like this is what would be needed:

% Index: lisp/progmodes/sh-script.el
% ===================================================================
% --- lisp/progmodes/sh-script.el (revision 45)
% +++ lisp/progmodes/sh-script.el (working copy)
% @@ -1438,7 +1438,7 @@
%                  ((and buffer-file-name
%                        (string-match "\\.m?spec\\'" buffer-file-name))
%                   "rpm")))))
% -    (unless interpreter
% +    (unless (or interpreter (null buffer-file-name))
%        (setq interpreter
%             (cond ((string-match "[.]sh\\>" buffer-file-name)
%                    "sh")

I haven't had a chance to build a snapshot with this change yet,
but has anyone else used sh-mode and seen this?  Does the change
look reasonable?

- Giorgos