Potential crash in PooledCurlHandleFactory

[coryan]

Reported anonymously. This:

google-cloud-cpp/google/cloud/internal/curl_handle_factory.cc

Lines 182 to 184 in 0ba6d2a

	auto const release_count =
	(std::min)(handles_.size() - maximum_size_ / 2,
	active_multi_handles_ - maximum_size_);

should be:

    auto const release_count =
        (std::min)(multi_handles_.size() - maximum_size_ / 2,
                   active_multi_handles_ - maximum_size_);

Notes the change from handles_ to multi_handles_ on line 183.