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.
A patch file is attached.