[hack] hphp/hack: fix clang exposed -Wint-conversion
[hack] hphp/hack: fix clang exposed -Wint-conversion

Summary: clang 3.7.0 exposed two -Wint-conversion violations:
```lang=bash
hphp/hack/src/heap/hh_shared.c:766:50: error: incompatible integer to pointer conversion passing 'size_t' (aka 'unsigned long') to parameter of type 'char *' [-Werror,-Wint-conversion]
  char* chunk       = __sync_fetch_and_add(heap, slot_size);
                                                 ^~~~~~~~~
hphp/hack/src/heap/hh_shared.c:808:64: error: incompatible integer to pointer conversion passing 'int' to parameter of type 'char *' [-Werror,-Wint-conversion]
     __sync_bool_compare_and_swap(&(hashtbl[slot].addr), NULL, 1)) {
                                                               ^
2 errors generated.
```

Reviewed By: int3

Differential Revision: D2617283

fb-gh-sync-id: 458faa1feb412b4889b6e4b7830a6bc098aaf487

chenglou/flow,facebook/flow,facebook/flow,chenglou/flow,nmote/flow,fletcherw/flow,claudiopro/flow,gabro/flow,ForbesLindesay/flow,cesarandreu/flow,jamesgpearce/flow,ForbesLindesay/flow,shiftcars/flow,ylu1317/flow,OCamlPro/flow,cesarandreu/flow,samwgoldman/flow,claudiopro/flow,fletcherw/flow,nmote/flow,cesarandreu/flow,AprilArcus/flow,facebook/flow,shiftcars/flow,graue/flow,popham/flow,samwgoldman/flow,d14b3l/flow,AprilArcus/flow,TiddoLangerak/flow,chenglou/flow,jgrund/flow,shiftcars/flow,jamesgpearce/flow,JonathanUsername/flow,jamesgpearce/flow,popham/flow,MichaelDeBoey/flow,fletcherw/flow,jeffmo/flow,jgrund/flow,AprilArcus/flow,graue/flow,AgentME/flow,popham/flow,jgrund/flow,jeffmo/flow,jamesgpearce/flow,MichaelDeBoey/flow,jgrund/flow,facebook/flow,claudiopro/flow,mroch/flow,jdelStrother/flow,graue/flow,gabro/flow,JonathanUsername/flow,mroch/flow,samwgoldman/flow,facebook/flow,gabelevi/flow,shiftcars/flow,claudiopro/flow,mroch/flow,shiftcars/flow,JonathanUsername/flow,gabelevi/flow,d14b3l/flow,AprilArcus/flow,jamesgpearce/flow,jdelStrother/flow,jeffmo/flow,jeffmo/flow,popham/flow,OCamlPro/flow,popham/flow,MichaelDeBoey/flow,mroch/flow,nmote/flow,d14b3l/flow,gabelevi/flow,jeffmo/flow,nmote/flow,AprilArcus/flow,samwgoldman/flow,MichaelDeBoey/flow,JonathanUsername/flow,jdelStrother/flow,ForbesLindesay/flow,jdelStrother/flow,TiddoLangerak/flow,ylu1317/flow,TiddoLangerak/flow,nmote/flow,TiddoLangerak/flow,AgentME/flow,nmote/flow,d14b3l/flow,d14b3l/flow,ylu1317/flow,gabro/flow,gabro/flow,gabelevi/flow,gabro/flow,gabelevi/flow,ForbesLindesay/flow,gabelevi/flow,facebook/flow,gabelevi/flow,jdelStrother/flow,popham/flow,jgrund/flow,popham/flow,ylu1317/flow,cesarandreu/flow,MichaelDeBoey/flow,fletcherw/flow,mroch/flow,fletcherw/flow,gabro/flow,jgrund/flow,ForbesLindesay/flow,jamesgpearce/flow,jdelStrother/flow,chenglou/flow,chenglou/flow,jamesgpearce/flow,samwgoldman/flow,graue/flow,claudiopro/flow,TiddoLangerak/flow,graue/flow,AgentME/flow,JonathanUsername/flow,graue/flow,d14b3l/flow,nmote/flow,mroch/flow,OCamlPro/flow,JonathanUsername/flow,AprilArcus/flow,OCamlPro/flow,mroch/flow,shiftcars/flow,ylu1317/flow,facebook/flow,samwgoldman/flow,AgentME/flow,chenglou/flow,samwgoldman/flow,ForbesLindesay/flow,JonathanUsername/flow,AgentME/flow,jeffmo/flow,fletcherw/flow