stream: Make stream_write_buffer() check for short writes

stream: Make stream_write_buffer() check for short writes

None of the calling sites to stream_write_buffer were checking the
return value to see if all bytes got written (nothing in current code
actually calls it any more after MEncoder was removed).

This was causing (very occasionally) problems with mencoder when using
output pipes AND running under a sandbox or when being straced (ptrace
is the culprit). Theoretically this problem can happen without pipes
or ptrace.

Only stream_file, stream_smb and stream_ffmpeg implement
write_buffer and ffmpeg already handles this internally.

Original patch by Sang-Uok Kum.

Signed-off-by: Tobias Diedrich <ranma@google.com>
git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@32881 b3059339-0415-0410-9bf9-f77b7e298cf2