avdevice/lavfi: fix self assignment warning

FAIL(ret) expands to statements including a silly ret=ret.
This triggers a -Wself-assign on confirmed clang 3.6, and so we fix it.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavdevice/lavfi.c

@@ -337,7 +337,7 @@ av_cold static int lavfi_read_header(AVFormatContext *avctx)
     }
 
     if ((ret = create_subcc_streams(avctx)) < 0)
-        FAIL(ret);
+        goto end;
 
     if (!(lavfi->decoded_frame = av_frame_alloc()))
         FAIL(AVERROR(ENOMEM));