If the length argument to apr_snprintf is 0, then we should return the
length that the string would be if we actually were going to fill it out.
However, if the length argument is 0, we can also accept a NULL string.
Also, added a test case for this.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@63829 13f79535-47bb-0310-9956-ffa450edef68
3 files changed