bug#14386: 24.3.50; `message-bury': (wrong-type-argument stringp nil)

[Drew Adams]

When I hit C-c C-c I got this backtrace.  This follows the recent fix of
bug #14085.
 
I noticed this with my setup, but also with emacs -Q.
 
Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  message-bury()
  message-send-and-exit(nil)
  call-interactively(message-send-and-exit nil nil)
  command-execute(message-send-and-exit)

In GNU Emacs 24.3.50.1 (i386-mingw-nt5.1.2600)
 of 2013-05-10 on ODIEONE
Bzr revision: 112542 rgm@gnu.org-20130510102119-fklj7xlajezey0tr
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.7) --no-opt --enable-checking --cflags
 -IC:/Devel/emacs/build/include --ldflags -LC:/Devel/emacs/build/lib'