[xcode11.3] [runtime] Don't zero-terminate after the string buffer. Fixes #7564.

[monojenkins]

Fixes #7564.

Backport of #7571.

/cc @mandel-macaque @rolfbjarne

[monojenkins]

Build failure
✅ Build succeeded
⚠️ Mono built from source
✅ API Diff (from stable)
✅ API Diff (from PR only) (no change)
✅ Generator Diff (no change)
🔥 Test run failed 🔥

Test results

3 tests failed, 84 tests passed.

Failed tests

• monotouch-test/iOS Unified 32-bits - simulator/Release (all optimizations): Failed
• monotouch-test/iOS Unified 64-bits - simulator/Debug: Failed
• monotouch-test/iOS Unified 64-bits - simulator/Debug (LinkSdk): Failed

[mandel-macaque]

Some networking issues that make the tests fail:

• [FAIL] ImageCaptioningTest.GetCaption : remote / no error -> https://github.com/xamarin/maccore/issues/2088
• [FAIL] UrlSessionTest.CreateDataTaskAsync : CreateDataTask a -> https://github.com/xamarin/maccore/issues/1939
• [FAIL] RegistrarTest.BlockCollection : System.ObjectDisposedException : Cannot access a disposed object. -> https://github.com/xamarin/maccore/issues/2089