avfilter/vf_libplacebo: also output a frame during input gaps

In constant FPS mode, it's possible for there no be *no* input active at
the current timestamp, even though there would be active inputs later in
the timeline (nb_active > 0). This would previously hit the AVERROR_BUG case.

With this change, we can instead output an empty frame.

libavfilter/vf_libplacebo.c

@@ -1243,6 +1243,10 @@ static int libplacebo_activate(AVFilterContext *ctx)
             }
         }
 
+        /* In constant FPS mode, we can also output an empty frame if there is
+         * a gap in the input timeline and we still have active streams */
+        ok |= s->fps.num && s->nb_active > 0;
+
         if (retry) {
             return 0;
         } else if (ok) {