swig-py: Fix a compiler warning about dead code.

* subversion/bindings/swig/core.i: For the typemap to handle buffer/len in
    args retrieve the length as an unsigned long and remove the pointless check
    for less than 0.  The temp variable is an apr_size_t which is unsigned and
    as such the test will never fail.  This shifts the bounds checking to
    Python's conversion code.  Unfortunately, in the PyInt case there's no way
    to do the bounds checking, but it's not like our existing bounds checking
    worked anyway.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1598174 13f79535-47bb-0310-9956-ffa450edef68