opus: Do not call vector_fmul_scalar on zero samples

The x86 variant of this function crashes in that specific case.

CC: libav-devel@libav.org

libavcodec/opusdec.c

@@ -587,7 +587,7 @@ static int opus_decode_packet(AVCodecContext *avctx, void *data,
             memset(frame->extended_data[i], 0, frame->linesize[0]);
         }
 
-        if (c->gain_i) {
+        if (c->gain_i && decoded_samples > 0) {
             c->fdsp.vector_fmul_scalar((float*)frame->extended_data[i],
                                        (float*)frame->extended_data[i],
                                        c->gain, FFALIGN(decoded_samples, 8));