fix violation of -Werror=sign-compare in MemorySchema
fix violation of -Werror=sign-compare in MemorySchema

Summary:
[thrift] fix violation of -Werror=sign-compare in MemorySchema
```
stderr: In file included from thrift\lib\cpp2\frozen\schema\MemorySchema.cpp:17:
In file included from buck-out/dev/gen/thrift/lib/cpp2/frozen/schema/memory_schema#header-mode-symlink-tree-only,headers\thrift/lib/cpp2/frozen/schema/MemorySchema.h:22:
buck-out/cells/fbsource/gen/xplat/third-party/glog/glogWindows#header-mode-symlink-tree-only,headers\glog/logging.h:700:32: error: comparison of integers of different signs: 'const unsigned long long' and 'const short' [-Werror,-Wsign-compare]
DEFINE_CHECK_OP_IMPL(Check_LE, <=)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
```

Reviewed By: mzlee

Differential Revision: D20353077

fbshipit-source-id: eca64e72f80e3b0afa13152df49c44c220d5c270

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift