Reformulate the uint64 value remapping code in packed_data.c such that
it no longer relies on undefined uint->int conversion behavior.

* subversion/libsvn_subr/packed_data.c
  (remap_uint,
   unmap_uint): New utilities factored out from the other functions.
                Reimplement the value mapping without using intermediate
                signed values.
  (svn_packed__data_flush_buffer,
   svn_packed__data_fill_buffer): Use the new utilities to pre / post
                                  process the buffered values.

Found by: philipm


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632623 13f79535-47bb-0310-9956-ffa450edef68