mjpegb: don't return 0 at the end of frame decoding.

Return 0 indicates "please return the same data again", i.e. it causes
an infinite loop. Instead, return that we consumed the buffer if we
finished decoding succesfully, or return an error if an error occurred.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/mjpegbdec.c

@@ -69,7 +69,7 @@ read_header:
     if (get_bits_long(&hgb, 32) != MKBETAG('m','j','p','g'))
     {
         av_log(avctx, AV_LOG_WARNING, "not mjpeg-b (bad fourcc)\n");
-        return 0;
+        return AVERROR_INVALIDDATA;
     }
 
     field_size = get_bits_long(&hgb, 32); /* field size */
@@ -149,7 +149,7 @@ read_header:
         picture->quality*= FF_QP2LAMBDA;
     }
 
-    return buf_ptr - buf;
+    return buf_size;
 }
 
 AVCodec ff_mjpegb_decoder = {