Fix an unitialized memory read reported by valgrind.  The reason was that
the entry struct type for the proto-index containes padding bytes (only
20 out of the 24 bytes are actually used by struct members). 

* subversion/libsvn_fs_x/index.c
  (svn_fs_x__l2p_proto_index_add_revision,
   svn_fs_x__l2p_proto_index_add_entry): Fully initialize the struct before
                                         writing it to the proto index file.
                                         Beware of padding!

Found by: philip

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1573237 13f79535-47bb-0310-9956-ffa450edef68