Followup to r1549924, fix lifetime bug that could lead to undefined
behaviour.  This is probably the cause of the Windows buildbot fails.

* subversion/libsvn_fs_fs/transaction.c
  (write_final_revprop): Increase scope/lifetime of stack variable.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1549947 13f79535-47bb-0310-9956-ffa450edef68