Add entry type checking to the P2L index reader to early detect
further cases external corruption.

* subversion/libsvn_fs_fs/index.c
  (read_entry): Verify that P2L index entries follow type restrictions.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1623373 13f79535-47bb-0310-9956-ffa450edef68