movenc: fix adpcm mono muxing.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/movenc.c

@@ -91,6 +91,7 @@ static int mov_write_stsz_tag(AVIOContext *pb, MOVTrack *track)
     }
     if (equalChunks) {
         int sSize = track->cluster[0].size/track->cluster[0].entries;
+        sSize = FFMAX(1, sSize); // adpcm mono case could make sSize == 0
         avio_wb32(pb, sSize); // sample size
         avio_wb32(pb, entries); // sample count
     }