Crash in Functions on MacOS when using authentication

### Please fill in the following fields:
Pre-built SDK from the [website](https://firebase.google.com/download/cpp)
Firebase C++ SDK version: 6.12.0
Firebase plugins in use (Auth, Database, etc.): Auth, Firestore, Functions, Storage
Additional SDKs you are using (Facebook, AdMob, etc.): Qt 5.14.1 clang 64bit
Platform you are using the C++ SDK on (Mac, Windows, or Linux): Mac
Platform you are targeting (iOS, Android, and/or desktop): desktop

### Please describe the issue here:

My application crashes when trying to call a Firebase callable function in combination with authentication:

auto pFireBaseApp = firebase::App::Create();
// No crash when I comment out the following line
auto pAuth = firebase::auth::Auth::GetAuth(pFireBaseApp);
auto pFunctions = functions::Functions::GetInstance(pFireBaseApp);

auto ref = pFunctions->GetHttpsCallable("testFunction");
auto future = ref.Call();
future.OnCompletion([](const Future &result) {
// Crash before this is reached but not when commenting out pAuth code
});

It does not crash, and behaves as expected, when not initializing Auth. Note that this behaves the same with a function that doesn't use authentication: `testFunction` is a very simple function that returns a fixed object.

This is the call stack of the crashed thread:

1 firebase::rest::BackgroundTransportCurl::PerformBackground(firebase::rest::Request *) (x86_64) /Users/jdierckx/work/Bioracer Motion/Bioracer Aero/build-BioracerAero-Desktop_Qt_5_14_1_clang_64bit-Debug/build-output/BRMApplicationTest.app/Contents/Frameworks/libBRMData.1.dylib 0x10038143b
2 firebase::rest::CurlThread::ProcessRequests() (x86_64) /Users/jdierckx/work/Bioracer Motion/Bioracer Aero/build-BioracerAero-Desktop_Qt_5_14_1_clang_64bit-Debug/build-output/BRMApplicationTest.app/Contents/Frameworks/libBRMData.1.dylib 0x10038318e
3 void * std::__thread_proxy>, void ( *)(void *), void *>>(void *) (x86_64) /Users/jdierckx/work/Bioracer Motion/Bioracer Aero/build-BioracerAero-Desktop_Qt_5_14_1_clang_64bit-Debug/build-output/BRMApplicationTest.app/Contents/Frameworks/libBRMData.1.dylib 0x1003a0cec
4 _pthread_start (x86_64) /usr/lib/system/libsystem_pthread.dylib 0x7fff64a29e65
5 thread_start (x86_64) /usr/lib/system/libsystem_pthread.dylib 0x7fff64a2583b

### Please answer the following, if applicable:
Have you been able to reproduce this issue with just the [Firebase C++ quickstarts](https://github.com/firebase/quickstart-cpp) ? Did not try (the quickstart example doesn't use Auth)

What's the issue repro rate? (eg 100%, 1/5 etc) 100%

Start by reproducing the sequence using App::Create, Auth::GetAuth, Functions::GetInstance, and HttpsCallable::Call on macOS. Inspect the crash path at BackgroundTransportCurl::PerformBackground and compare it with the Firebase C++ quickstart setup, noting that the reported quickstart does not include Auth. Done means callable Functions requests no longer crash when Auth is initialized.

Written by the indexing model from the issue text.