fix build with UBSAN
commit498bf807e4efa4c6233600d37931fc38ade5cb14
authorIgor Sugak <sugak@fb.com>
Thu, 16 Nov 2017 22:58:01 +0000 (16 14:58 -0800)
committerHhvm Bot <hhvm-bot@users.noreply.github.com>
Thu, 16 Nov 2017 23:09:45 +0000 (16 15:09 -0800)
tree59f1d7ef6118996ddb8291e93d4258b930701a0a
parent5ebc19412a22a741631cc8dbdaefcc5b4c995c5e
fix build with UBSAN

Summary:
```lang
Assertion failure: buck-out/dev/gen/hphp/util/alloc#header-mode-symlink-tree-with-header-map,headers/hphp/util/alloc.h:344: int HPHP::mallctlHelper(const char *, T *, T *, bool) [T
= unsigned int]: assertion `mallctl != nullptr' failed.
```

Reviewed By: edwinsmith, meyering

Differential Revision: D6348354

fbshipit-source-id: 1e23aac7b880da94f662fb42f82228062fd124de
hphp/util/alloc.h