SyntaxError__str__(): Fix two memory problems discovered by Insure.

SyntaxError__str__(): Fix two memory problems discovered by Insure.
First, the allocated buffer was never freed after using it to create
the PyString object.  Second, it was possible that have_filename would
be false (meaning that filename was not a PyString object), but that
the code would still try to PyString_GET_SIZE() it.