bug#1990: 23.0.60; Emacs trying too hard to find fonts that don't exist

[Chong Yidong]

Juanma Barranquero <lekktu@gmail.com> writes:

> For me, it fixes #1952: moving in etc/HELLO is again fast.
>
> It does not fix #1990, though. Emacs is still repeatedly trying to
> find fonts for characters that do not have one:
>
> 9 matches for "1501" in buffer: *Help*
>      28:font for: (1501 . hebrew-iso8859-8)

That log message is emitted just before calling fontset_font (which is
where we would check whether fonts are known not to exist).