Some of our FSFS config settings will cause malfunction if invalid
values were given.  Thus, sanitize them before using them.

* subversion/libsvn_fs_fs/fs_fs.c
  (sanitize_block_size): New function to verify config data.
  (read_config): Use the new function to guarantee valid settings
                 in our FS struct.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628392 13f79535-47bb-0310-9956-ffa450edef68