Close sets the internal stream pointer to NULL

As recommended in https://lists.columbia.edu/pipermail/portaudio/2017-March/001076.html

Author: mgeier

sounddevice.py

@@ -1286,6 +1286,7 @@ def close(self, ignore_errors=True):
 
         """
         err = _lib.Pa_CloseStream(self._ptr)
+        self._ptr = _ffi.NULL
         if not ignore_errors:
             _check(err, 'Error closing stream')