espeak_ng_Cancel(): discard the audio, don't close the device!

This function should just discard the currently playing audio,
rather than close the audio device.  After all,
we will probably want to synthesize something else later.

src/libespeak-ng/speech.c

 
 #ifdef HAVE_PCAUDIOLIB_AUDIO_H
 	if ((my_mode & ENOUTPUT_MODE_SPEAK_AUDIO) == ENOUTPUT_MODE_SPEAK_AUDIO)
-		audio_object_close(my_audio);
+		audio_object_flush(my_audio);
 #endif
 	embedded_value[EMBED_T] = 0; // reset echo for pronunciation announcements