pthread: don't increment index on zero-sized packets.

The next call to decode() will update from an invalid index, which will
either lead to a memcpy() where dest==src (2 threads), or lead to a
crash (>2 threads).
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/pthread.c

@@ -491,6 +491,7 @@ static int submit_packet(PerThreadContext *p, AVPacket *avpkt)
     }
 
     fctx->prev_thread = p;
+    fctx->next_decoding++;
 
     return 0;
 }
@@ -513,8 +514,6 @@ int ff_thread_decode_frame(AVCodecContext *avctx,
     err = submit_packet(p, avpkt);
     if (err) return err;
 
-    fctx->next_decoding++;
-
     /*
      * If we're still receiving the initial packets, don't return a frame.
      */