perf_to_profile crashes on a 3GB perf.data file

```
perf record ./src/bitcoind -datadir=mainnet -server=0 -connect=127.0.0.1 -dbcache=16000
```

I ran the above command to capture a perf of a long running process (3-4 hours) and got the following error:

```
[libprotobuf FATAL external/com_google_protobuf/src/google/protobuf/stubs/stringpiece.cc:50] size too big: 18446744072542064660 details: string length exceeds max size
terminate called after throwing an instance of 'google::protobuf::FatalException'
what(): size too big: 18446744072542064660 details: string length exceeds max size
fish: Job 1, '../perf_data_converter/result/b…' terminated by signal SIGABRT (Abort)
```

The `perf.data` file generated by the `perf` was 3GB. It's too big to attach in the github issues but I can provide it on request.

EDIT: I just saw #123 and should mention that the binary I was using was built on `b1c2b82a8ec75b016c36da31c845491c75b9adae`