Add NUL terminator to string object size calculation; ref #5032

mrbgems/mruby-os-memsize/src/memsize.c

@@ -54,6 +54,7 @@ os_memsize_of_object(mrb_state* mrb, mrb_value obj)
       size += mrb_objspace_page_slot_size();
       if (!RSTR_EMBED_P(RSTRING(obj)) && !RSTR_SHARED_P(RSTRING(obj))) {
         size += RSTRING_CAPA(obj);
+        size++; /* NUL terminator */
       }
       break;
     case MRB_TT_CLASS: