Be consistent in the way we create file / stream data buffers.

* subversion/libsvn_fs_fs/transaction.c
  (fnv1a_checksum_on_file_range): Allocate the data buffer from the
                                  APR pool instead of from stack.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1588899 13f79535-47bb-0310-9956-ffa450edef68