PATCH: Fix unset PATH crash (tracker #4315)

PATCH: Fix unset PATH crash

Per Masamichi Hosada:
On Windows, lilypond crashes when the environment variable PATH is not set.

```
C:\tmp\lilypond-2.19.16-0.mingw\$_OUTDIR\usr\bin>set PATH=

C:\tmp\lilypond-2.19.16-0.mingw\$_OUTDIR\usr\bin>lilypond
GNU LilyPond 2.19.16
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct null not valid

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

C:\tmp\lilypond-2.19.16-0.mingw\$_OUTDIR\usr\bin>
```

Even in the case of linux, this issue is a possible crash.