ICU-20041 Improve handling of OOM failures in NumberingSystem class. (#19)

ICU-20041 Improve handling of OOM failures in NumberingSystem class. (#19)

ICU-20041 ICU4C NumberingSystem class doesn't handle out-of-memory (OOM) failures.

- Not all code paths in the NumberingSystem class check for OOM failures. This can lead to crashes in some cases as null pointers will be dereferenced without any checks.
- Change to use nullptr instead of NULL.
- Don't stomp on OOM errors when attempting to load resources. We should report back OOM to the caller.
- Use LocalPointer in order simplify the code and for automatic clean-up of memory.
- Use LocalUResourceBundlePointer as well to help simply things even more.