Merge commit 'd1ad85fc' into release/2.4

* commit 'd1ad85fc':
  vf_format: check input validity

See: ee16e0caMerged-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_format.c

@@ -60,8 +60,10 @@ static av_cold int init(AVFilterContext *ctx)
     int i;
     int ret;
 
-    if (!s->pix_fmts)
+    if (!s->pix_fmts) {
+        av_log(ctx, AV_LOG_ERROR, "Empty output format string.\n");
         return AVERROR(EINVAL);
+    }
 
     /* count the formats */
     cur = s->pix_fmts;