code cleanup: remove unnecessary current_voice_id in readclause.c

Instead of passing a temporary variable current_voice_id to SSML
processing logic and then copying the value to voice_change, pass
voice_change directly. GetVoiceAttributes() both sets voice_change and
retuns CLAUSE_TYPE_VOICE_CHANGE that is used by TranslateClause() for
error checking.

tests/ssml/language-switch.ssml covers the intended behavior of language
switching and will fail when either voice_change or CLAUSE_TYPE_VOICE_CHANGE
is not set correctly.

src/libespeak-ng/readclause.c

 
 static espeak_VOICE base_voice;
 static char base_voice_variant_name[40] = { 0 };
-static char current_voice_id[40] = { 0 };
 
 static int n_param_stack;
 PARAM_STACK param_stack[N_PARAM_STACK];
 	tr->clause_upper_count = 0;
 	tr->clause_lower_count = 0;
 	*tone_type = 0;
-	*voice_change = 0;
 
 	if (ungot_char2 != 0)
 		c2 = ungot_char2;
 					xml_buf[n_xml_buf] = 0;
 					c2 = ' ';
 
-					terminator = ProcessSsmlTag(xml_buf, buf, &ix, n_buf, xmlbase, &audio_text, current_voice_id, &base_voice, base_voice_variant_name, &ignore_text, &clear_skipping_text, &sayas_mode, &sayas_start, ssml_stack, &n_ssml_stack, &n_param_stack, (int *)speech_parameters);
+					terminator = ProcessSsmlTag(xml_buf, buf, &ix, n_buf, xmlbase, &audio_text, voice_change, &base_voice, base_voice_variant_name, &ignore_text, &clear_skipping_text, &sayas_mode, &sayas_start, ssml_stack, &n_ssml_stack, &n_param_stack, (int *)speech_parameters);
 
 					if (terminator != 0) {
 						buf[ix] = ' ';
 						buf[ix++] = 0;
-
-						if (terminator & CLAUSE_TYPE_VOICE_CHANGE)
-							strcpy(voice_change, current_voice_id);
 						return terminator;
 					}
 					c1 = ' ';
 
 	option_punctuation = speech_parameters[espeakPUNCTUATION];
 	option_capitals = speech_parameters[espeakCAPITALS];
-
-	current_voice_id[0] = 0;
-
 	ignore_text = false;
 	audio_text = false;
 	clear_skipping_text = false;

src/libespeak-ng/ssml.c

 	// a voice change.
 	// If it's a closing tag, delete the top frame of the stack and determine whether this implies
 	// a voice change.
-	// Returns  CLAUSE_TYPE_VOICE_CHANGE if there is a voice change
+	// if there is a voice change, sets current_voice_id and returns CLAUSE_TYPE_VOICE_CHANGE
 
 	wchar_t *lang;
 	wchar_t *gender;