Fix bad usage of uint8_fast_t in Back_Scan_Utf8_Char
Fix bad usage of uint8_fast_t in Back_Scan_Utf8_Char

The purpose of the `uint_fast8_t` type in `<stdint.h>` is to allow
you to express that you only need 8 bits worth of storage, but the
compiler is allowed to decide that the fastest way it can manipulate
small quantities on the platform is by using a size larger than 8-bits.

https://stackoverflow.com/q/35055042/

Hence it may not be the same size as an `unsigned char`, and should
not be used to do things like address into strings (for example).

A mistake in the usage of uint_fast8_t that meant to use it to hold a
trailing byte count was actually using it on the wrong line, which
was making it try to point at characters.  This leads to crashes on
platforms where sizeof(uint8_fast_t) != sizeof(unsigned char)...
for example, OpenBSD.

Fix is to use the uint_fast8_t on the right line.

draegtun/ren-c,draegtun/ren-c,draegtun/ren-c,rgchris/ren-c,rgchris/ren-c,giuliolunati/ren-c,hostilefork/rebol,hostilefork/rebol,giuliolunati/ren-c,rgchris/ren-c,rgchris/ren-c,hostilefork/rebol,hostilefork/rebol,draegtun/ren-c,draegtun/ren-c,rgchris/ren-c,giuliolunati/ren-c,hostilefork/rebol,hostilefork/rebol,draegtun/ren-c,giuliolunati/ren-c,giuliolunati/ren-c