lavdev/lavfi: correctly set the inout pad index when linking a filter to the output sink

In lavfi_read_header(), use the pad index designated in the inout for
linking an output to a sink, rather than always 0. Fix link creation
for filters with more than one output (e.g. the split filter).

libavdevice/lavfi.c

@@ -178,7 +178,7 @@ av_cold static int lavfi_read_header(AVFormatContext *avctx,
                                                 pix_fmts, lavfi->graph)) < 0)
             FAIL(ret);
         lavfi->sinks[i] = sink;
-        if ((ret = avfilter_link(inout->filter_ctx, 0, sink, 0)) < 0)
+        if ((ret = avfilter_link(inout->filter_ctx, inout->pad_idx, sink, 0)) < 0)
             FAIL(ret);
     }