avcodec/qpeg: Limit copy in qpeg_decode_intra() to the available bytes

Fixes: Timeout (27 sec -> 39 milli sec)
Fixes: 13151/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_QPEG_fuzzer-5717536023248896

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/qpeg.c

@@ -99,6 +99,8 @@ static void qpeg_decode_intra(QpegContext *qctx, uint8_t *dst,
                 }
             }
         } else {
+            if (bytestream2_get_bytes_left(&qctx->buffer) < copy)
+                copy = bytestream2_get_bytes_left(&qctx->buffer);
             for(i = 0; i < copy; i++) {
                 dst[filled++] = bytestream2_get_byte(&qctx->buffer);
                 if (filled >= width) {