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.