[PATCH] Fix filename encoding error in svn_io_open_unique_file3()

I spotted a filename encoding error when using "delete on pool cleanup"
with svn_io_open_unique_file3(). I have not tested this, but I believe
this bug would fail to clean up the temp file if the UTF-8 encoding of
the directory path differs from its APR encoding.

How can I easily test this in my Ubuntu UTF-8 environment?

- Julian