Memory leak in CompactTrace::extract

```
$ hhvm --version
HipHop VM 3.30.2 (rel)
Compiler: 1547606131_793769619
Repo schema: f64db376aeddd959936df38c14d823991dc434d9

$ hh_client --version
hh-fa0e3c70afe06e7b6566bb2b4e3edde95563addc-3.30.2 Tue Jan 15 17:54:19 2019
```
This is running on a in-house built package on RHEL6.

We haven't yet been able to reproduce this memory leak outside of production, but the behavior we observe is that whenever a request comes in that is particularly long-lived, memory usage on the host increases at a very fast and constant rate. Turning jemalloc profiling on and taking periodic heap dumps, we see that all the memory gets sucked up by this path:


Fortunately, its very easy to avoid the offending path by setting `Eval.EnableCompactBacktrace` to false in conf (the memory leak doesn't reappear in prod), but this would've been a showstopper otherwise.