lavd/lavfi: fix compiler warning for uninitialized variables

Signed-off-by: Jean First <jeanfirst@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

libavdevice/lavfi.c

@@ -270,7 +270,8 @@ static int lavfi_read_packet(AVFormatContext *avctx, AVPacket *pkt)
     int stream_idx, min_pts_sink_idx = 0;
     AVFilterBufferRef *ref;
     AVPicture pict;
-    int ret, i, size;
+    int ret, i;
+    int size = 0;
 
     /* iterate through all the graph sinks. Select the sink with the
      * minimum PTS */