Fixes #591 by eliminating infinite loop

Changes proposed in this pull request:

• Added return statement in merge function after the elifs to ensure that recursive combinations are resolved properly. Any situation not explicitly handled by the existing conditions will be covered by the added return statement, which will prevent the merge logic from entering an undefined or recursive state.
• Added tests in tests_am.py to ensure the bug was fixed as well as added additional tests to ensure proper handling of large numbers and ones with irregular splits in the Amharic language.

Status

• READY
• HOLD
• WIP (Work-In-Progress)

How to verify this change

Test with num2words(568476685, lang="am") or "num2words 568476685 -l am" in the command line. Can also run the test_am.py file to check with test case/assert statement.

Additional notes

Fixed Issue #591