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>