Fix some logic in PythonCmd() -- if the AsString() call failed, we'd be calling LEAVE_PYTHON twice (once in PythonCmd_Error() and once in PythonCmd() itself).
Fix some logic in PythonCmd() -- if the AsString() call failed, we'd
be calling LEAVE_PYTHON twice (once in PythonCmd_Error() and once in
PythonCmd() itself).

This fix doesn't fix the IDLE problems, but at least makes them more
debuggable by getting rid of the segfault.

Will backport.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator