[compiler-rt] Handle None value when polling addr2line pipe

According to:
https://docs.python.org/3/library/subprocess.html#subprocess.Popen.poll

poll can return None if the process hasn't terminated.

I'm not quite sure how addr2line could end up closing the pipe without
terminating but we did see this happen on one of our bots:
```
<...>scripts/asan_symbolize.py",
line 211, in symbolize
    logging.debug("addr2line exited early (broken pipe), returncode=%d"
% self.pipe.poll())
TypeError: %d format: a number is required, not NoneType
```

Handle None by printing a message that we couldn't get the return
code.

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D101891