Assertion failure in CurlResource::closeForSweep()

**Describe the bug**
We are seeing a small volume of HHVM process segfaults with the following c++ stack:
```
__GI___assert_fail at assert.c:101
HPHP::CurlResource::closeForSweep at /build/hhvm/hphp/runtime/ext/curl/curl-resource.cpp:152
HPHP::CurlResource::close at /build/hhvm/hphp/runtime/ext/curl/curl-resource.cpp:138
HPHP::f_curl_close at /build/hhvm/hphp/runtime/ext/curl/ext_curl.cpp:468
```

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

We do not have a repro.

**Expected behavior**

No segfault.

**Actual behavior**

Process segfault.

**Environment**
- Operating system
```
cy@slack-www-hhvm-main-iad-yxcv:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.7 LTS
Release: 16.04
Codename: xenial
```
- Installation method
Slack production installation.
- HHVM Version
```
root@slack-www-hhvm-main-iad-yxcv:/var/www/html/slack# hhvm --version
HipHop VM 4.56.4 (rel)
Compiler: remotes/origin/4.56.4-slack-patch1-0-g06a1ba129ade4f70f866712711cfeebe16294c53
Repo schema: 020f58e2a994da2aaaf4f4234558ac335ab0ed1f
```