bug#5984: Crash displaying composed characters

[Stefan Monnier]

>   . Emacs then enters redisplay to display the echo area.  As part of
[...]
>   . Further down, autocmp_chars calls the value of
>     auto-composition-function:
[...]
>   . Now the " *Echo Area0*" buffer holds a totally different text,
>     unbeknownst to autocmp_chars, which still passes the old values 32
>     and 33 to TEMP_SET_PT_BOTH:

More generally, this Lisp code could modify any buffer, so preventing
the load-messages is not a sufficiently reliable solution (tho it might
be desirable in any case).


        Stefan