fix the bracketed linearization in Python after the introduction of nonExist and BIND

src/runtime/python/pypgf.c

@@ -1603,7 +1603,9 @@ static PgfLinFuncs pgf_bracket_lin_funcs = {
 	.symbol_token  = pgf_bracket_lzn_symbol_token,
 	.expr_literal  = pgf_bracket_lzn_expr_literal,
 	.begin_phrase  = pgf_bracket_lzn_begin_phrase,
-	.end_phrase    = pgf_bracket_lzn_end_phrase
+	.end_phrase    = pgf_bracket_lzn_end_phrase,
+	.symbol_ne     = NULL,
+	.symbol_bind   = NULL
 };
 
 static PyObject*