pthreads: reset active_thread_type when slice thread_init returrns early

(cherry picked from commit f77f640b30)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pthread.c

@@ -239,8 +239,10 @@ static int thread_init(AVCodecContext *avctx)
     ThreadContext *c;
     int thread_count = avctx->thread_count;
 
-    if (thread_count <= 1)
+    if (thread_count <= 1) {
+        avctx->active_thread_type = 0;
         return 0;
+    }
 
     c = av_mallocz(sizeof(ThreadContext));
     if (!c)