avfilter/vf_shufflepixels: Check ff_get_video_buffer()

There would be a segfault in case of (likely memory allocation) failure.
Fixes Coverity issue #1322338.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/vf_shufflepixels.c

@@ -377,6 +377,11 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     ThreadData td;
     int ret;
 
+    if (!out) {
+        ret = AVERROR(ENOMEM);
+        goto fail;
+    }
+
     ret = av_frame_copy_props(out, in);
     if (ret < 0) {
         av_frame_free(&out);