Make fallthrough explicit in tok_parse.c

In response to #325

src/mwparserfromhell/parser/ctokenizer/tok_parse.c

@@ -1558,6 +1558,9 @@ Tokenizer_handle_tag_close_close(Tokenizer *self)
             Py_XDECREF(so);
             Py_XDECREF(sc);
         }
+        // One of `so` or `sc` failed to allocate so we fall through to clean up the
+        // closing tag and propagate the error by returning NULL.
+        __attribute__((fallthrough));
         case -1:
             Py_DECREF(closing);
             return NULL;