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>
