avcodec: properly check pkt_timebase for validity

Unset/invalid timebases have a zero numerator.
This makes the checks consistent with other timebase checks and fixes an
integer division by 0.

libavcodec/libzvbi-teletextdec.c

@@ -412,7 +412,7 @@ static int teletext_decode_frame(AVCodecContext *avctx, void *data, int *data_si
         }
     }
 
-    if (avctx->pkt_timebase.den && pkt->pts != AV_NOPTS_VALUE)
+    if (avctx->pkt_timebase.num && pkt->pts != AV_NOPTS_VALUE)
         ctx->pts = av_rescale_q(pkt->pts, avctx->pkt_timebase, AV_TIME_BASE_Q);
 
     if (pkt->size) {

libavcodec/utils.c

@@ -2435,7 +2435,7 @@ int avcodec_decode_subtitle2(AVCodecContext *avctx, AVSubtitle *sub,
         } else {
             avctx->internal->pkt = &pkt_recoded;
 
-            if (avctx->pkt_timebase.den && avpkt->pts != AV_NOPTS_VALUE)
+            if (avctx->pkt_timebase.num && avpkt->pts != AV_NOPTS_VALUE)
                 sub->pts = av_rescale_q(avpkt->pts,
                                         avctx->pkt_timebase, AV_TIME_BASE_Q);
             ret = avctx->codec->decode(avctx, sub, got_sub_ptr, &pkt_recoded);