smoothstreamingenc: explict cast to avoid overflow

CC: libav-stable@libav.org
Bug-Id: CID 732248

libavformat/smoothstreamingenc.c

@@ -576,7 +576,7 @@ static int ism_write_packet(AVFormatContext *s, AVPacket *pkt)
     SmoothStreamingContext *c = s->priv_data;
     AVStream *st = s->streams[pkt->stream_index];
     OutputStream *os = &c->streams[pkt->stream_index];
-    int64_t end_dts = (c->nb_fragments + 1) * c->min_frag_duration;
+    int64_t end_dts = (c->nb_fragments + 1) * (int64_t) c->min_frag_duration;
     int ret;
 
     if (st->first_dts == AV_NOPTS_VALUE)