dirstate: fix parse_dirstate() to error out if NULL entry created

Since 83f0e93ec34b "dirstate-item: move the C implementation to the same
logic", dirstate_item_from_v1_data() gets more likely to return NULL, and
the fuzzer crashes because of that.

mercurial/cext/parsers.c

@@ -770,6 +770,8 b' static PyObject *parse_dirstate(PyObject'
 
 		entry = (PyObject *)dirstate_item_from_v1_data(state, mode,
 		                                               size, mtime);
+		if (!entry)
+			goto quit;
 		cpos = memchr(cur, 0, flen);
 		if (cpos) {
 			fname = PyBytes_FromStringAndSize(cur, cpos - cur);