Tealer error in check_rekey_to

When running the command `tealer contract.teal`, I get the following error:
```
Traceback (most recent call last):
File "/Users/gidon/tealer/venv/bin/tealer", line 33, in
sys.exit(load_entry_point('tealer==0.0.2', 'console_scripts', 'tealer')())
File "/Users/gidon/tealer/venv/lib/python3.8/site-packages/tealer-0.0.2-py3.8.egg/tealer/__main__.py", line 75, in main
File "/Users/gidon/tealer/venv/lib/python3.8/site-packages/tealer-0.0.2-py3.8.egg/tealer/detectors/rekeyto.py", line 79, in detect
File "/Users/gidon/tealer/venv/lib/python3.8/site-packages/tealer-0.0.2-py3.8.egg/tealer/detectors/rekeyto.py", line 74, in check_rekey_to
File "/Users/gidon/tealer/venv/lib/python3.8/site-packages/tealer-0.0.2-py3.8.egg/tealer/detectors/rekeyto.py", line 74, in check_rekey_to
File "/Users/gidon/tealer/venv/lib/python3.8/site-packages/tealer-0.0.2-py3.8.egg/tealer/detectors/rekeyto.py", line 74, in check_rekey_to
[Previous line repeated 18 more times]
File "/Users/gidon/tealer/venv/lib/python3.8/site-packages/tealer-0.0.2-py3.8.egg/tealer/detectors/rekeyto.py", line 54, in check_rekey_to
KeyError: 4
```
I haven't managed to isolate the breaking code (the teal code is over 2000 lines) so I am not sure what the issue is.

When I run the command with the `--print-cfg` option then it does successfully create the CFG.

Reproduce the failure with `tealer contract.teal` and inspect `tealer/detectors/rekeyto.py`, especially the recursive `check_rekey_to` path reported in the traceback. Compare this with the successful `--print-cfg` run and isolate a smaller input that still raises `KeyError: 4`; done means the detector handles that input without the exception and the regression is covered.

Written by the indexing model from the issue text.