ftp: fix flush control connection input

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>

libavformat/ftp.c

@@ -138,7 +138,7 @@ static int ftp_flush_control_input(FTPContext *s)
     s->conn_control_block_flag = 1;
     do {
         err = ftp_get_line(s, buf, sizeof(buf));
-    } while (err > 0);
+    } while (!err);
 
     s->conn_control_block_flag = ori_block_flag;