Fixing duplicate SSH authentication failure events.

We now do not raise more than one failure event per connection.

Addresses BIT-1641.

testing/btest/scripts/base/protocols/ssh/one-auth-fail-only.test

@@ -0,0 +1,7 @@
+# @TEST-EXEC: bro -C -r $TRACES/ssh/sshguess.pcap %INPUT | sort | uniq -c >output
+# @TEST-EXEC: btest-diff output
+
+event ssh_auth_failed(c: connection)
+    {
+    print c$uid;
+    }