crash starting hhvm in server mode if hhvm.log.level set to a valid value

### HHVM Version

2018.08.08 nightly
3.27.1
3.24.x

### Operating System and Version

MacOS High Sierra

### Standalone code, or other way to reproduce the problem

```
hhvm --no-config -m server -p 8080 -d hhvm.log.level=Warning
```

'warning' is fine.

### Actual result

```
Failed to determine jemalloc background thread state

Assertion failure: /tmp/hhvm-nightly-20180808-7263-1w735qv/hhvm-nightly-2018.08.08/hphp/runtime/base/memory-manager.cpp:756: void *HPHP::MemoryManager::newSlab(size_t): assertion `reinterpret_cast(mem) % kSlabAlign == 0' failed.

-------------------------------Assertion Failure--------------------------------
/tmp/hhvm-nightly-20180808-7263-1w735qv/hhvm-nightly-2018.08.08/hphp/runtime/base/memory-manager.cpp:756: void *HPHP::MemoryManager::newSlab(size_t): assertion `reinterpret_cast(mem) % kSlabAlign == 0' failed.
Core dumped: Abort trap: 6
Stack trace in /tmp/stacktrace.53465.log
Abort trap: 6
```

### Expected result

Server starts