Fix `Symbol#size` for multi-byte characters with `MRB_UTF8_STRING`

Before:

  p :あ.size  #=> 3

After:

  p :あ.size  #=> 1

1 files changed, 7 insertions, 0 deletions

diff --git a/mrbgems/mruby-symbol-ext/src/symbol.c b/mrbgems/mruby-symbol-ext/src/symbol.c
index a992dbfce..215226502 100644
--- a/mrbgems/mruby-symbol-ext/src/symbol.c
+++ b/mrbgems/mruby-symbol-ext/src/symbol.c
@@ -1,6 +1,7 @@
 #include <mruby.h>
 #include <mruby/khash.h>
 #include <mruby/array.h>
+#include <mruby/string.h>
 
 typedef struct symbol_name {
   size_t len;
@@ -45,7 +46,13 @@ static mrb_value
 mrb_sym_length(mrb_state *mrb, mrb_value self)
 {
   mrb_int len;
+#ifdef MRB_UTF8_STRING
+  mrb_int byte_len;
+  const char *name = mrb_sym2name_len(mrb, mrb_symbol(self), &byte_len);
+  len = mrb_utf8_len(name, byte_len);
+#else
   mrb_sym2name_len(mrb, mrb_symbol(self), &len);
+#endif
   return mrb_fixnum_value(len);
 }