bpo-42206: Propagate and raise errors from PyAST_Validate in the parser

Misc/NEWS.d/next/Core and Builtins/2020-10-30-13-11-01.bpo-42206.xxssR8.rst

@@ -0,0 +1,2 @@
+Propagate and raise the errors caused by :c:func:`PyAST_Validate` in the
+parser.

Parser/pegen.c

@@ -1143,7 +1143,9 @@ _PyPegen_run_parser(Parser *p)
         p->start_rule == Py_file_input ||
         p->start_rule == Py_eval_input)
     {
-        assert(PyAST_Validate(res));
+        if (!PyAST_Validate(res)) {
+            return NULL;
+        }
     }
 #endif
     return res;