avformat/ftp: Solve a crash bug when network occur a exception

This fixes a proble where ffmpeg would cause crash to do a seek when the network disconnect.
The log like this:
01-01 10:53:03.441  6580  6580 F DEBUG   : backtrace:
01-01 10:53:03.441  6580  6580 F DEBUG   :     #00 pc 0002942e  /system/lib/libavformat.so (ffurl_write+9)
Signed-off-by: tiejun.peng <tiejun.peng@foxmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/ftp.c

@@ -206,6 +206,9 @@ static int ftp_send_command(FTPContext *s, const char *command,
     if (response)
         *response = NULL;
 
+    if (!s->conn_control)
+        return AVERROR(EIO);
+
     if ((err = ffurl_write(s->conn_control, command, strlen(command))) < 0)
         return err;
     if (!err)