Fix a bug in VP9Worker which leads to unit test hang.

This fixes the hang in VP9/InvalidFileTest.ReturnCode/3
due to worker->had_error has not been reset after getting
error.

Change-Id: Ia3608225094758a2bd88f6ae4dd9dfd93bbaad27

vp9/decoder/vp9_thread.c

@@ -107,6 +107,7 @@ int vp9_worker_reset(VP9Worker* const worker) {
 }
 
 void vp9_worker_execute(VP9Worker* const worker) {
+  worker->had_error = 0;
   if (worker->hook != NULL) {
     worker->had_error |= !worker->hook(worker->data1, worker->data2);
   }