Make v4l_read_header() don't free a stream in case of failure when

reading its header. The stream will be freed later in
av_open_input_stream(). Fix a segmentation fault due to a double free
on the same pointer.

Originally committed as revision 14246 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavdevice/v4l.c

@@ -258,7 +258,6 @@ static int grab_read_header(AVFormatContext *s1, AVFormatParameters *ap)
  fail:
     if (video_fd >= 0)
         close(video_fd);
-    av_free(st);
     return AVERROR(EIO);
 }