string.c (chars2bytes): small refactoring

src/string.c

@@ -330,16 +330,15 @@ chars2bytes(mrb_value s, mrb_int off, mrb_int idx)
     return idx;
   }
   else {
-    mrb_int b = 0;
-    const char *p = RSTRING_PTR(s) + off;
+    const char *p0 = RSTRING_PTR(s) + off;
+    const char *p = p0;
     const char *e = RSTRING_END(s);
-
-    for (mrb_int i=0; p<e && i<idx; i++) {
-      mrb_int n = mrb_utf8len(p, e);
-      b += n;
-      p += n;
+    while (p < e && idx--) {
+      p += mrb_utf8len(p, e);
     }
-    return b;
+    mrb_int len = (mrb_int)(p-p0);
+    if (idx > 0 || p > e) len++;
+    return len;
   }
 }