Set thread_count in the decoder before to open it.

This is safer since avcodec_open() may initialize some structure using
the thread_count default value which will be changed later, causing
potential security / stability issues.

Also do not explicitely set the thread_count value in the decoder,
since this is already done by avcodec_thread_init().

See the thread:
"[FFmpeg-devel] [PATCH] Make ffplay define in the encoder the number of threads *before* to open it",
started in date 2008-03-08.

Originally committed as revision 20954 to svn://svn.ffmpeg.org/ffmpeg/trunk
