Handle <frozen importlib._bootstrap> filepath

[anderoonies]

prevents:

 .../site-packages/debug_toolbar_line_profiler/panel.py", line 80, in func_std_string
    file_path, file_name = file_name.rsplit(os.sep, 1)
ValueError: not enough values to unpack (expected 2, got 1)

[mikekeda]

This fix is included in #21