Fix test on invalid null will topic value in connect packet

Signed-off-by: Jerome Malinge <gromgromm@yahoo.com>

test/broker/07-will-null-topic.py

@@ -3,6 +3,8 @@
 import struct
 
 from mosq_test_helper import *
+from socket import error as SocketError
+import errno
 
 rc = 1
 keepalive = 60
@@ -16,6 +18,11 @@ try:
     sock = mosq_test.do_client_connect(connect_packet, b"", timeout=30, port=port)
     rc = 0
     sock.close()
+except SocketError as e:
+    if e.errno == errno.ECONNRESET:
+        # Connection has been closed by peer (very quickly).
+        # Fine, this is the expected behavior.
+        rc = 0
 finally:
     broker.terminate()
     broker.wait()