avcodec/dcadec: Search and decode frame in case it starts later in a packet

This fixes decoding the first frame of some dts files
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dcadec.c

@@ -1487,8 +1487,11 @@ static int dca_decode_frame(AVCodecContext *avctx, void *data,
     s->exss_ext_mask = 0;
     s->xch_present   = 0;
 
-    s->dca_buffer_size = avpriv_dca_convert_bitstream(buf, buf_size, s->dca_buffer,
-                                                  DCA_MAX_FRAME_SIZE + DCA_MAX_EXSS_HEADER_SIZE);
+    s->dca_buffer_size = AVERROR_INVALIDDATA;
+    for (i = 0; i < buf_size - 3 && s->dca_buffer_size == AVERROR_INVALIDDATA; i++)
+        s->dca_buffer_size = avpriv_dca_convert_bitstream(buf + i, buf_size - i, s->dca_buffer,
+                                                          DCA_MAX_FRAME_SIZE + DCA_MAX_EXSS_HEADER_SIZE);
+
     if (s->dca_buffer_size == AVERROR_INVALIDDATA) {
         av_log(avctx, AV_LOG_ERROR, "Not a valid DCA frame\n");
         return AVERROR_INVALIDDATA;