ffmpeg: Fix memory corruption with ffserver
Bug has been introduced by:
commit 1cede1d0112b5b26e6d023846729662b7cf1853f
Author: Anton Khirnov <anton@khirnov.net>
Date:   Sat Jun 25 07:43:49 2011 +0200

    ffmpeg: factor common code from new_a/v/s/d_stream to new_output_stream()
