avconv: when using -loop option bail out if seek to start fails

Fixes an infinite loop when a demuxer fails to seek to the start of the input.
Signed-off-by: Peter Große <pegro@friiks.de>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

avtools/avconv.c

@@ -2615,8 +2615,10 @@ static int process_input(void)
         return ret;
     }
     if (ret < 0 && ifile->loop) {
-        if ((ret = seek_to_start(ifile, is)) < 0)
-            return ret;
+        ret = seek_to_start(ifile, is);
+        if(ret < 0)
+            av_log(NULL, AV_LOG_WARNING, "Seek to start failed.\n");
+        else
             ret = get_input_packet(ifile, &pkt);
     }
     if (ret < 0) {