Clip number of thread to 16 when using threaded decoding in avcodec.

It workarounds an avcodec issue.
(cherry picked from commit e587ce36)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/avcodec/video.c

@@ -337,6 +337,7 @@ int InitVideoDec( decoder_t *p_dec, AVCodecContext *p_context,
     int i_thread_count = var_InheritInteger( p_dec, "ffmpeg-threads" );
     if( i_thread_count <= 0 )
         i_thread_count = vlc_GetCPUCount();
+    i_thread_count = __MIN( i_thread_count, 16 );
     msg_Dbg( p_dec, "allowing %d thread(s) for decoding", i_thread_count );
     p_sys->p_context->thread_count = i_thread_count;
 #endif