avcodec/utils: Force mutex to NULL after destruction.

A badly behaving user provided mutex manager (such as that in OpenCV) may not reset the mutex to NULL on destruction.  This can cause a problem for a later mutex manager (which may assert that the mutex is NULL before creating).
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -3461,6 +3461,8 @@ int av_lockmgr_register(int (*cb)(void **mutex, enum AVLockOp op))
             return -1;
         if (lockmgr_cb(&avformat_mutex, AV_LOCK_DESTROY))
             return -1;
+        codec_mutex = NULL;
+        avformat_mutex = NULL;
     }
 
     lockmgr_cb = cb;