avcodec/tiff: Fix loop detection

Fixes regression with tickets/4364/L1004220.DNG

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/tiff.c

@@ -1747,7 +1747,7 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *p,
                         int *got_frame, AVPacket *avpkt)
 {
     TiffContext *const s = avctx->priv_data;
-    unsigned off, last_off;
+    unsigned off, last_off = 0;
     int le, ret, plane, planes;
     int i, j, entries, stride;
     unsigned soff, ssize;
@@ -1812,7 +1812,6 @@ again:
     /** whether we should process this multi-page IFD's next page */
     retry_for_page = s->get_page && s->cur_page + 1 < s->get_page;  // get_page is 1-indexed
 
-    last_off = off;
     if (retry_for_page) {
         // set offset to the next IFD
         off = ff_tget_long(&s->gb, le);
@@ -1830,6 +1829,7 @@ again:
             avpriv_request_sample(s->avctx, "non increasing IFD offset");
             return AVERROR_INVALIDDATA;
         }
+        last_off = off;
         if (off >= UINT_MAX - 14 || avpkt->size < off + 14) {
             av_log(avctx, AV_LOG_ERROR, "IFD offset is greater than image size\n");
             return AVERROR_INVALIDDATA;