Merge r1643437 from trunk:

 * r1643437
   Correctly reject requests with invalid virtual transaction names.
   Justification:
     These requests aren't valid and should result in an error.
   Votes:
     +1: breser, philip, kotkov




git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1643455 13f79535-47bb-0310-9956-ffa450edef68