avcodec/g729dec: require buf_size to be non 0

The 0 case was added with the support for multiple packets. It
appears unintended and causes extra complexity and out of array
accesses (though within padding)

No testcase

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit f64be9da)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/g729dec.c

@@ -416,7 +416,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame_ptr,
         return ret;
     out_frame = (int16_t*) frame->data[0];
 
-    if (buf_size % 10 == 0) {
+    if (buf_size && buf_size % 10 == 0) {
         packet_type = FORMAT_G729_8K;
         format = &format_g729_8k;
         //Reset voice decision