Fix a memleak with win32 threads: the handle returned by _beginthreadex

must be closed (this differs from _beginthread).
Patch by Shehzad Salim (shehzadsalim gmail com)

Originally committed as revision 19633 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/w32thread.c

@@ -69,6 +69,7 @@ void avcodec_thread_free(AVCodecContext *s){
         WaitForSingleObject(c[i].thread, INFINITE);
         if(c[i].work_sem) CloseHandle(c[i].work_sem);
         if(c[i].done_sem) CloseHandle(c[i].done_sem);
+        if(c[i].thread)   CloseHandle(c[i].thread);
     }
 
     av_freep(&s->thread_opaque);