Fix the test failure

Summary: AllocateFromHugePage() can return nullptr, and then we need to try to allocate the block with AllocateNewBlock()

Test Plan: arena_test

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D47607

util/arena.cc

@@ -90,8 +90,7 @@ char* Arena::AllocateFallback(size_t bytes, bool aligned) {
     block_head = AllocateFromHugePage(size);
   }
 #endif
-  if (size == 0) {
-    assert(block_head == nullptr);
+  if (!block_head) {
     size = kBlockSize;
     block_head = AllocateNewBlock(size);
   }