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.

src/include/sys-char.h

@@ -198,12 +198,12 @@ inline static const REBYTE *Back_Scan_UTF8_Char(
 ){
     *out = 0;
 
-    const uint_fast8_t *source = bp;
-    REBCNT trail = trailingBytesForUTF8[*source];
+    const REBYTE *source = bp;
+    uint_fast8_t trail = trailingBytesForUTF8[*source];
 
     // Check that we have enough valid source bytes:
     if (size) {
-        if (trail + 1 > *size)
+        if (cast(uint_fast8_t, trail + 1) > *size)
             return nullptr;
     }
     else if (trail != 0) {