avformat/icodec: Change order of operations to avoid NULL dereference

Fixes: SEGV on unknown address 0x000000000000
Fixes: 26379/clusterfuzz-testcase-minimized-ffmpeg_dem_ICO_fuzzer-5709011753893888

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Peter Ross
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/icodec.c

@@ -156,12 +156,14 @@ static int read_packet(AVFormatContext *s, AVPacket *pkt)
     IcoDemuxContext *ico = s->priv_data;
     IcoImage *image;
     AVIOContext *pb = s->pb;
-    AVStream *st = s->streams[0];
+    AVStream *st;
     int ret;
 
     if (ico->current_image >= ico->nb_images)
         return AVERROR_EOF;
 
+    st = s->streams[0];
+
     image = &ico->images[ico->current_image];
 
     if ((ret = avio_seek(pb, image->offset, SEEK_SET)) < 0)