Check event_count before merging blocks

Author: colesbury

THCCachingAllocator.cpp

@@ -276,7 +276,7 @@ struct THCCachingAllocator
  /** combine previously split blocks */
  void try_merge_blocks(Block* dst, Block* src, FreeBlocks& free_blocks)
  {
- if (!src || src->allocated) {
+ if (!src || src->allocated || src->event_count > 0) {
  return;
  }
  if (dst->prev == src) {
@@ -386,6 +386,7 @@ struct THCCachingAllocator
  if (err != cudaSuccess) return err;
 
  std::set<THCStreamPtr> streams(std::move(block->stream_uses));
+ THAssert(block->stream_uses.empty());
  for (auto it = streams.begin(); it != streams.end(); ++it) {
  auto& stream = *it;