Fix crash on multi-thread decoding.

[tyan0]

When decoding:
https://download.blender.org/peach/bigbuckbunny_movies/big_buck_bunny_1080p_h264.mov in multi-threads, openh264 crashes with segfault. This patch fixes the issue.

How to reproduce:

• Download https://download.blender.org/peach/bigbuckbunny_movies/big_buck_bunny_1080p_h264.mov
• Run ffmpeg -i big_buck_bunny_1080p_h264.mov -t 10 -vcodec copy -an bbb.264
• Enable multi-thread in h264dec by:

diff --git a/codec/console/dec/src/h264dec.cpp b/codec/console/dec/src/h264dec.cpp
index 59365844..4338bbce 100644
--- a/codec/console/dec/src/h264dec.cpp
+++ b/codec/console/dec/src/h264dec.cpp
@@ -624,7 +624,7 @@ int32_t main (int32_t iArgC, char* pArgV[]) {
     pDecoder->SetOption (DECODER_OPTION_TRACE_LEVEL, &iLevelSetting);
   }

-  int32_t iThreadCount = 0;
+  int32_t iThreadCount = 3;
   pDecoder->SetOption (DECODER_OPTION_NUM_OF_THREADS, &iThreadCount);

   if (pDecoder->Initialize (&sDecParam)) {

• Build openh264.
• Run ./h264dec bbb.264 x.yuv
• This crashes with segfault.