Fix leaked completions (leading to leaked calls in thread_stress_test)

src/core/surface/completion_queue.c

@@ -116,7 +116,7 @@ void grpc_cq_begin_op(grpc_completion_queue *cc) {
 void grpc_cq_end_op(grpc_completion_queue *cc, void *tag, int success,
                     void (*done)(void *done_arg, grpc_cq_completion *storage),
                     void *done_arg, grpc_cq_completion *storage) {
-  int shutdown = gpr_unref(&cc->pending_events);
+  int shutdown;
 
   storage->tag = tag;
   storage->done = done;
@@ -124,15 +124,15 @@ void grpc_cq_end_op(grpc_completion_queue *cc, void *tag, int success,
   storage->next =
       ((gpr_uintptr)&cc->completed_head) | ((gpr_uintptr)(success != 0));
 
+  gpr_mu_lock(GRPC_POLLSET_MU(&cc->pollset));
+  shutdown = gpr_unref(&cc->pending_events);
   if (!shutdown) {
-    gpr_mu_lock(GRPC_POLLSET_MU(&cc->pollset));
     cc->completed_tail->next =
         ((gpr_uintptr)storage) | (1u & (gpr_uintptr)cc->completed_tail->next);
     cc->completed_tail = storage;
     grpc_pollset_kick(&cc->pollset);
     gpr_mu_unlock(GRPC_POLLSET_MU(&cc->pollset));
   } else {
-    gpr_mu_lock(GRPC_POLLSET_MU(&cc->pollset));
     cc->completed_tail->next =
         ((gpr_uintptr)storage) | (1u & (gpr_uintptr)cc->completed_tail->next);
     cc->completed_tail = storage;