fbuffer.h: expand arguments

* ext/json/fbuffer/fbuffer.h (fbuffer_to_s): expand arguments, for
  fix with rb_str_new macro.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47626 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/json/fbuffer/fbuffer.h

@@ -181,7 +181,7 @@ static FBuffer *fbuffer_dup(FBuffer *fb)
 
 static VALUE fbuffer_to_s(FBuffer *fb)
 {
-    VALUE result = rb_str_new(FBUFFER_PAIR(fb));
+    VALUE result = rb_str_new(FBUFFER_PTR(fb), FBUFFER_LEN(fb));
     fbuffer_free(fb);
     FORCE_UTF8(result);
     return result;