string.c (bytes2chars): skip calling mrb_utf8len() if possible

If (ch < 0x80) the length of the character (in bytes) should be 1, so we
don't have to call mrb_utf8len().

src/string.c

@@ -358,7 +358,8 @@ bytes2chars(mrb_value s, mrb_int bi)
   mrb_int i;
 
   for (i = 0; p < pivot; i++) {
-    p += mrb_utf8len(p, e);
+    if ((*p & 0x80) == 0) p++;
+    else p += mrb_utf8len(p, e);
   }
   if (p != pivot) return -1;
   return i;