Always call avcodec_thread_init()

The various avcodec_thread_init() functions are updated to return
immediately after setting avctx->thread_count.  This allows -threads 0
to pass through to codecs.  It also simplifies the usage for apps
using libavcodec.

Originally committed as revision 21358 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/os2thread.c

@@ -116,6 +116,9 @@ int avcodec_thread_init(AVCodecContext *s, int thread_count){
 
     s->thread_count= thread_count;
 
+    if (thread_count <= 1)
+        return 0;
+
     assert(!s->thread_opaque);
     c= av_mallocz(sizeof(ThreadContext)*thread_count);
     s->thread_opaque= c;