bug#3809: 23.1.50; crash in abbrev mode

[Chong Yidong]

> My Emacs (compiled with CFLAGS='-g -DENABLE_CHECKING') has
> just crashed as this:
>
> % emacs -Q
> C-x m
> M-x message-mode
> handa@
>
> Emacs crashes when I type the last "@" (in To: subject
> line).  It seems that "handa" can be any other string.  It
> is reproducible with EMACS_23_1_RC branch too.

I can't reproduce this, even with CFLAGS='-g -DENABLE_CHECKING'.  But,
you're right about this:

> In the above code, at least we should have a ckeck of
> SYMBOLP (sym).

Could you go ahead and add this check, and if this fixes the crash for
you, please close the bug.  Thanks.