libvisual/lv_audio.c: Address warning -Wuninitialized

Symptom was:
> ../../libvisual-0.4.2/libvisual/lv_audio.c:1111:2: error: variable 'signedcorr' is uninitialized when used here [-Werror,-Wuninitialized]
>         signedcorr += byte_max_numeric (size) / 2;
>         ^~~~~~~~~~

Author: hartwork

libvisual/libvisual/lv_audio.c

@@ -1053,7 +1053,7 @@ static int transform_format_buffer_from_float (VisBuffer *dest, VisBuffer *src,
 	int signedcorr;
 	int i;
 
-	signedcorr += byte_max_numeric (size) / 2;
+	signedcorr = byte_max_numeric (size) / 2;
 
 	if (size == 1)
 		FORMAT_BUFFER_FROM_FLOAT(int8_t, uint8_t)