pthread_frame: fix uninitialized variable read

Could lead to random behavior. This possibly happened due to commit
32a5b63126. This should/could probably be simplified, but for no apply
a minimal fix to quell the errors.

Tested-by: Michael Niedermayer <michael@niedermayer.cc>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pthread_frame.c

@@ -468,7 +468,7 @@ int ff_thread_decode_frame(AVCodecContext *avctx,
     FrameThreadContext *fctx = avctx->internal->thread_ctx;
     int finished = fctx->next_finished;
     PerThreadContext *p;
-    int err, ret;
+    int err, ret = 0;
 
     /* release the async lock, permitting blocked hwaccel threads to
      * go forward while we are in this function */