avformat/movenc: Check av_malloc()

Fixes Coverity issue #1596735.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit 601873263e)

libavformat/movenc.c

@@ -1222,6 +1222,8 @@ static int mov_write_chnl_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
     if (ret || !config) {
         config = 0;
         speaker_pos = av_malloc(layout->nb_channels);
+        if (!speaker_pos)
+            return AVERROR(ENOMEM);
         ret = ff_mov_get_channel_positions_from_layout(layout,
                 speaker_pos, layout->nb_channels);
         if (ret) {