Merge pull request #8461 from haberman/ruby-arena-memory-leak

Fixed memory leak of Ruby arena objects.

ruby/ext/google/protobuf_c/protobuf.c

@@ -180,6 +180,7 @@ static void Arena_mark(void *data) {
 static void Arena_free(void *data) {
   Arena *arena = data;
   upb_arena_free(arena->arena);
+  xfree(arena);
 }
 
 static VALUE cArena;