bad interaction between set-buffer-multibyte and narrowing

[Dave Love]

This is cut down from a real example in a Gnus bug report.  Note how
the buffer-string is truncated.

Making the buffer unibyte leaves point-max unchanged and it gets
reduced when the buffer is made multibyte again.  You can see this by
just making a narrowed buffer unibyte which contains Japanese.  The
effect is present in both the released and development code.

(with-temp-buffer
  (insert "From: ニシキノネット　FC事業部
Content-Type: text/plain; charset=ISO-2022-JP\n\n")
  (narrow-to-region (point-min) (1- (point-max)))
  (set-buffer-multibyte nil)
  (set-buffer-multibyte t)
  (buffer-string))
  => "From: ニシキノネット　FC事業部
Content-Type: text/plain"