Return ENS_SPEECH_STOPPED from Synthesize when the speech callback returns >1 to stop the audio.

src/include/espeak-ng/espeak_ng.h

 	ENS_UNSUPPORTED_PHON_FORMAT  = 0x10000BFF,
 	ENS_NO_SPECT_FRAMES          = 0x10000CFF,
 	ENS_EMPTY_PHONEME_MANIFEST   = 0x10000DFF,
+	ENS_SPEECH_STOPPED           = 0x10000EFF,
 } espeak_ng_STATUS;
 
 typedef enum {

src/libespeak-ng/espeak_api.c

 	switch (status)
 	{
 	case ENS_OK:                     return EE_OK;
+	case ENS_SPEECH_STOPPED:         return EE_OK;
 	case ENS_VOICE_NOT_FOUND:        return EE_NOT_FOUND;
 	case ENS_MBROLA_NOT_FOUND:       return EE_NOT_FOUND;
 	case ENS_MBROLA_VOICE_NOT_FOUND: return EE_NOT_FOUND;

src/libespeak-ng/speech.c

 			finished = synth_callback((short *)outbuf, length, event_list);
 		if (finished) {
 			SpeakNextClause(NULL, 0, 2); // stop
-			break;
+			return ENS_SPEECH_STOPPED;
 		}
 
 		if (Generate(phoneme_list, &n_phoneme_list, 1) == 0) {
 							return ENS_AUDIO_ERROR;
 					} else if (synth_callback)
 						synth_callback(NULL, 0, event_list); // NULL buffer ptr indicates end of data
-					break;
+					return ENS_OK;
 				}
 			}
 		}
 	}
-	return ENS_OK;
 }
 
 void MarkerEvent(int type, unsigned int char_position, int value, int value2, unsigned char *out_ptr)