Make sure all contents of the FSFS f7 indexes are fully deterministic.

For internal convenience, the FSFS format7 P2L index describes the rev
contents in full blocks of whatever granularity.  The last block will
usually extend beyond EOF and we add an "empty / unused" padding entry
for it to the index.

It's checksum is never used nor verified but it should not report random
values.

* subversion/libsvn_fs_fs/index.c
  (svn_fs_fs__p2l_index_append): Use 0 as checksum of the P2L index filler
                                 entry as we do for all other empty regions.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606142 13f79535-47bb-0310-9956-ffa450edef68
* subversion/libsvn_fs_fs/index.c
  (svn_fs_fs__p2l_index_append): Use symbolic constant instead of a
                                 hardcoded value.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606144 13f79535-47bb-0310-9956-ffa450edef68