This is a follow up to commit e7e5ddd2. oss-fuzz found a case where
performance was pathologically bad with a long `stop_chars` string.
Since our inner loop in that case was iterating over `stop_chars` and
comparing each of them to `buffer[i]`, we can use `memchr()` the
opposite way round to in commit e7e5ddd2 to speed that up, using
`buffer[i]` as the needle in a `stop_chars` haystack.

From some brief testing, this doesn’t impact on the performance of a
more normal use case of having a short (<10 bytes long) `stop_chars`. I
was slightly concerned that the function call overhead of calling out to
`memchr()` would have an impact there, but apparently not.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

oss-fuzz#372994443