backport.pl: fix infinite loop if
~/.subversion/auth/svn.simple/d3c8a345b14f6a1b42251aef8027ab57 didn't exist.

* tools/dist/backport.pl
  ($AVAILID): Error-check open()[1], simplify logic while here.

[1] That's one of the things I _really_ like in Python compared to Perl..

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507667 13f79535-47bb-0310-9956-ffa450edef68