Fix a null pointer dereference in DoVoiceChange(synthesize.c). [Coverity]

src/libespeak-ng/synthesize.c

 #include "config.h"
 
 #include <ctype.h>
+#include <errno.h>
 #include <math.h>
 #include <stdint.h>
 #include <stdio.h>
 }
 #endif
 
-void DoVoiceChange(voice_t *v)
+espeak_ng_STATUS DoVoiceChange(voice_t *v)
 {
 	// allocate memory for a copy of the voice data, and free it in wavegenfill()
 	voice_t *v2;
-
-	v2 = (voice_t *)malloc(sizeof(voice_t));
+	if ((v2 = (voice_t *)malloc(sizeof(voice_t))) == NULL)
+		return ENOMEM;
 	memcpy(v2, v, sizeof(voice_t));
 	wcmdq[wcmdq_tail][0] = WCMD_VOICE;
 	wcmdq[wcmdq_tail][2] = (intptr_t)v2;
 	WcmdqInc();
+	return ENS_OK;
 }
 
 void DoEmbedded(int *embix, int sourceix)

src/libespeak-ng/voice.h

 espeak_VOICE *SelectVoiceByName(espeak_VOICE **voices, const char *name);
 voice_t *LoadVoice(const char *voice_name, int control);
 voice_t *LoadVoiceVariant(const char *voice_name, int variant);
-void DoVoiceChange(voice_t *v);
+espeak_ng_STATUS DoVoiceChange(voice_t *v);
 void WVoiceChanged(voice_t *wvoice);
 void WavegenSetVoice(voice_t *v);
 void ReadTonePoints(char *string, int *tone_pts);