Prevent NPE in TcpSyslogMessageSender#close (#83)

Co-authored-by: Amir Wiener <awiener@fortinet.com>

Author: AmirWiener

src/main/java/com/cloudbees/syslog/sender/TcpSyslogMessageSender.java

@@ -288,6 +288,8 @@ public String toString() {
 
     @Override
     public void close() throws IOException {
-        this.socket.close();
+        if (socket != null) {
+            this.socket.close();
+        }
     }
 }