Avoid failing to chown non-existent Unix-domain sockets

Changing ownership of sockets that doesn't exist isn't a great
practice.

A better approach would be to apply os.chown() to the file descriptor
of the open socket, but at the very least dkimpy-milter shouldn't
crash the way it currently does if the socket isn't already present.

dkimpy_milter/util.py

@@ -160,6 +160,7 @@ def own_socketfile(milterconfig):
         offset = 5
 
     if offset is not None:
+        if os.path.exists(sockname[offset:]):
             os.chown(sockname[offset:], user, group)