Cygwin: cwdstuff::get: clean up debug_printf output

Set errno = 0 at the beginning so that the debug_printf call at the
end doesn't report a nonzero errno left over from some other function
call.

winsup/cygwin/path.cc

@@ -5022,6 +5022,8 @@ char *
 cwdstuff::get (char *buf, int need_posix, int with_chroot, unsigned ulen)
 {
   tmp_pathbuf tp;
+
+  errno = 0;
   if (ulen)
     /* nothing */;
   else if (buf == NULL)