threads: allow thread count of zero

This moves setting the thread count to a minimum of 1 to
frame_thread_init(), allowing a value of zero to propagate
through to the codec if frame threading is not used.  This
makes auto-threads work in libx264.

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit ff1efc524c)

libavcodec/pthread.c

@@ -683,6 +683,11 @@ static int frame_thread_init(AVCodecContext *avctx)
     FrameThreadContext *fctx;
     int i, err = 0;
 
+    if (thread_count <= 1) {
+        avctx->active_thread_type = 0;
+        return 0;
+    }
+
     avctx->thread_opaque = fctx = av_mallocz(sizeof(FrameThreadContext));
 
     fctx->threads = av_mallocz(sizeof(PerThreadContext) * thread_count);
@@ -882,8 +887,6 @@ int ff_thread_init(AVCodecContext *avctx, int thread_count)
         return -1;
     }
 
-    avctx->thread_count = FFMAX(1, thread_count);
-
     if (avctx->codec) {
         validate_thread_parameters(avctx);