Fix UnixSyslog message ident corruption (#20)

According to openlog(3) documentation:

    if the string *ident points to ceases to exist,
    the results are undefined". So we have to
    keep the identBuffer during the existence of
    the Syslog object.

Closes #21

src/main/java/org/graylog2/syslog4j/impl/unix/UnixSyslog.java

@@ -37,6 +37,8 @@ public class UnixSyslog extends AbstractSyslog {
         public void closelog();
     }
 
+    protected static Memory identBuffer = null;
+
     protected static int currentFacility = -1;
     protected static boolean openlogCalled = false;
 
@@ -81,8 +83,6 @@ public class UnixSyslog extends AbstractSyslog {
                     ident = null;
                 }
 
-                Memory identBuffer = null;
-
                 if (ident != null) {
                     identBuffer = new Memory(128);
                     identBuffer.setString(0, ident, false);