wavegen.c: Fix -Wunused-but-set-variable warning.

src/libespeak-ng/wavegen.c

@@ -1397,13 +1397,14 @@ static int SpeedUp(short *outbuf, int length_in, int length_out, int end_of_text
 int WavegenFill(void)
 {
 	int finished;
-	unsigned char *p_start;
-
-	p_start = out_ptr;
 
 	finished = WavegenFill2();
 
 #if HAVE_SONIC_H
+	unsigned char *p_start;
+
+	p_start = out_ptr;
+
 	if (sonicSpeed > 1.0) {
 		int length;
 		int max_length;