bug#11282: 24.1.50; Error in decoding message

[Richard Stallman <rms@gnu.org>]

This bug report will be sent to the Bug-GNU-Emacs mailing list
and the GNU bug tracker at debbugs.gnu.org.  Please check that
the From: line contains a valid email address.  After a delay of up
to one day, you should receive an acknowledgement at that address.

Please write in English if possible, as the Emacs maintainers
usually do not have translators for other languages.

Please describe exactly what actions triggered the bug, and
the precise symptoms of the bug.  If you can, give a recipe
starting from `emacs -Q':

When I visit this as an Rmail file, Rmail gets an error
decoding the coding system.  Maybe the message really is
invalid, but Rmail should not get an unhandled Lisp error.

Here is the backtrace.

    Debugger entered--Lisp error: (wrong-type-argument wholenump nil)
      decode-coding-region(888 1351 utf-7)
      rmail-mime-insert-decoded-text([("text/html" (charset . "utf-7")) ("inline") "7bit" [[nil nil nil] [t nil t]] [#<marker at 1 in bad-message> #<marker at 5343 in bad-message> t] ["" (nil) nil] [#<marker at 5343 in bad-message> #<marker at 5806 in bad-message> t] nil rmail-mime-insert-text nil])
      rmail-mime-insert-text([("text/html" (charset . "utf-7")) ("inline") "7bit" [[nil nil nil] [t nil t]] [#<marker at 1 in bad-message> #<marker at 5343 in bad-message> t] ["" (nil) nil] [#<marker at 5343 in bad-message> #<marker at 5806 in bad-message> t] nil rmail-mime-insert-text nil])
      rmail-mime-insert([("text/html" (charset . "utf-7")) ("inline") "7bit" [[nil nil nil] [t nil t]] [#<marker at 1 in bad-message> #<marker at 5343 in bad-message> t] ["" (nil) nil] [#<marker at 5343 in bad-message> #<marker at 5806 in bad-message> t] nil rmail-mime-insert-text nil])
      rmail-show-mime()
      rmail-show-message-1(nil)
      rmail-show-message(nil)
      byte-code("\bq\210	\204\f