avcodec/cavsdec: print warning if no frame has been decoded

Fixes ticket3128
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/cavsdec.c

@@ -1167,8 +1167,11 @@ static int cavs_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
     buf_end = buf + buf_size;
     for(;;) {
         buf_ptr = avpriv_find_start_code(buf_ptr, buf_end, &stc);
-        if ((stc & 0xFFFFFE00) || buf_ptr == buf_end)
+        if ((stc & 0xFFFFFE00) || buf_ptr == buf_end) {
+            if (!h->stc)
+                av_log(h->avctx, AV_LOG_WARNING, "no frame decoded\n");
             return FFMAX(0, buf_ptr - buf);
+        }
         input_size = (buf_end - buf_ptr) * 8;
         switch (stc) {
         case CAVS_START_CODE: