Depend on the known behaviour of URLs for nonexistent files rather than
the undefined behaviour of URLs for directories (which fails on Windows).
The test isn't about file: URLs at all, so the URL used doesn't really
matter.
If a % ended the statement, the string's trailing NUL would be skipped
and memory past the end of the buffer would be accessed and potentially
displayed as part of the --write-out output. Added tests 1440 and 1441
to check for this kind of condition.
Reported-by: Brian Carpenter