bpo-18407: ast.c uses Py_ssize_t for asdl_seq_LEN() iterator (GH-10655)

When iterating using asdl_seq_LEN(), use 'Py_ssize_t' type instead of
'int' for the iterator variable, to avoid downcast on 64-bit platforms.

_Py_asdl_int_seq_new() now also ensures that the index is greater than
or equal to 0.

Include/asdl.h

@@ -36,7 +36,7 @@ asdl_int_seq *_Py_asdl_int_seq_new(Py_ssize_t size, PyArena *arena);
     do { \
         Py_ssize_t _asdl_i = (I); \
         assert((S) != NULL); \
-        assert(_asdl_i < (S)->size); \
+        assert(0 <= _asdl_i && _asdl_i < (S)->size); \
         (S)->elements[_asdl_i] = (V); \
     } while (0)
 #else