bpo-37072: Fix crash in PyAST_FromNodeObject() when flags is NULL (#13634)

I'm confident that this fixes the reported crash. flags=NULL is treated as using the latest minor version.

https://bugs.python.org/issue37072

Misc/NEWS.d/next/Core and Builtins/2019-05-28-18-18-55.bpo-37072.1Hewl3.rst

+Fix crash in PyAST_FromNodeObject() when flags is NULL.
\ No newline at end of file

Python/ast.c

@@ -786,7 +786,7 @@ PyAST_FromNodeObject(const node *n, PyCompilerFlags *flags,
     /* borrowed reference */
     c.c_filename = filename;
     c.c_normalize = NULL;
-    c.c_feature_version = flags->cf_feature_version;
+    c.c_feature_version = flags ? flags->cf_feature_version : PY_MINOR_VERSION;
 
     if (TYPE(n) == encoding_decl)
         n = CHILD(n, 0);