rotatelogs: Remove another use of a consant length buffer for errors.

* support/rotatelogs.c
  (doRotate): Use apr_psprintf() and %pm.  Move the destruction of the pool
    after we're done with the error message so the error string stays allocated
    long enough.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1534896 13f79535-47bb-0310-9956-ffa450edef68