ffmpeg: dont print "Conversion failed" if conversion has never started

Found-by: jamrial
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -517,7 +517,7 @@ static void ffmpeg_cleanup(int ret)
     if (received_sigterm) {
         av_log(NULL, AV_LOG_INFO, "Received signal %d: terminating.\n",
                (int) received_sigterm);
-    } else if (ret) {
+    } else if (ret && transcode_init_done) {
         av_log(NULL, AV_LOG_INFO, "Conversion failed!\n");
     }
     term_exit();