parsers: set exception when there's too little string data to extract parents

Previously we were returning NULL from this function without actually
setting up an exception. This fixes that problem, which was detected
with cpychecker.

mercurial/parsers.c

@@ -481,8 +481,11 b' static PyObject *parse_dirstate(PyObject'
 	len = readlen;
 
 	/* read parents */
-	if (len < 40)
+	if (len < 40) {
+		PyErr_SetString(
+			PyExc_ValueError, "too little data for parents");
 		goto quit;
+	}
 
 	parents = Py_BuildValue("s#s#", str, 20, str + 20, 20);
 	if (!parents)