ensure 'sock' is always defined (pedantic)

xpra/net/socket_util.py

@@ -785,6 +785,7 @@ def timeout_probe(sockpath):
                         del e
     except Exception:
         for sock_def in defs.keys():
+            sock = str(sock_def)
             try:
                 sock = f"{sock_def[0]} {sock_def[2]!r}"
                 cleanup_socket = sock_def[-1]