mprintf: allow %.s with data not being zero terminated

If the precision is indeed shorter than the string, don't strlen() to
find the end because that's not how the precision operator works.

I also added a unit test for curl_msnprintf to make sure this works and
that the fix doesn't a few other basic use cases. I found a POSIX
compliance problem that I marked TODO in the unit test, and I figure we
need to add more tests in the future.

Reported-by: Török Edwin