Possibly fixed error

[Henrik Grubbstr�m (Lysator) @ Pike (-) developers forum]

I suspect that this behavior is another aspect of [bug 7723], and was
fixed with commit 57ebfa38c979402b6bdca02ab6593e570d963342:

| commit 57ebfa38c979402b6bdca02ab6593e570d963342
| Author: Henrik Grubbström (Grubba) <gru...@grubba.org>
| Date:   Fri Jun 17 17:34:34 2016 +0200
| 
|     combine_path: Survive string_builder without NUL-terminations.
|     
|     The string_builder API no longer maintains NUL-termination of the
|     buffer. This caused combine_path_nt() et al to fail intermittently
|     due to reading past the buffer end.
|     
|     Also adds a corresponding test.
|     
|     Fixes [bug 7723].