Pyperf crashes when run in verbose mode

Running 'PyPerf -v' after building from source crashes with the following output:
```
terminate called after throwing an instance of 'std::logic_error'
what(): basic_string::_M_construct null not valid
Aborted (core dumped)
```
uname -a:
`Linux ubuntu 4.15.0-121-generic #123-Ubuntu SMP Mon Oct 5 16:16:40 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux`

Start by reproducing the source-built PyPerf invocation with the `-v` flag on the reported Ubuntu/Linux environment and compare the crash output. Trace the verbose-mode entry point to identify the null string construction, then verify that PyPerf no longer crashes when run in verbose mode.

Written by the indexing model from the issue text.