Fix compiler warning in libsvn_subr C tests for packed data.

* subversion/tests/libsvn_subr/packed-data-test.c
  (unpack_subs, unpack): Cast the types that are narrower in the structures
    than the pack stream get functions return.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570419 13f79535-47bb-0310-9956-ffa450edef68