pp_reverse: Use isUTF8_CHAR() for validity check

The code was converting a UTF-8 single character string to its code
point, just to find out if the string was wellformed or not.  It had no
need of the returned code point.  It is more efficient to just use the
function that checks for validity without trying to get the code point.

The validity checking function returns 0 on failure; else how many bytes
were in the string.  Use this value instead of calling UTF8SKIP to
re-derive it.

And the former function utf8_to_uvchr_buf() is discouraged from use
anyway.  This commit removes the final core use of the discouraged
_uvchr functions,

Author: khwilliamson

pp.c

@@ -6636,10 +6636,11 @@ PP_wrapped(pp_reverse, 0, 1)
                         continue;
                     }
                     else {
-                        if (!utf8_to_uvchr_buf(s, send, 0))
+                        Size_t advance = isUTF8_CHAR(s, send);
+                        if (advance == 0)
                             break;
                         up = (char*)s;
-                        s += UTF8SKIP(s);
+                        s += advance;
                         down = (char*)(s - 1);
                         /* reverse this character */
                         while (down > up) {