[tcp] Add missing null check in resolveDomain()
The method lookupHostAddress() returns null in case of an error, hence
we need to test if the returned value is null prior adding the endpoint.
Should fix the following NPE:
java.lang.NullPointerException:
at org.jivesoftware.smack.tcp.XMPPTCPConnection.connectUsingConfiguration (XMPPTCPConnection.java:606)
at org.jivesoftware.smack.tcp.XMPPTCPConnection.connectInternal (XMPPTCPConnection.java:846)
at org.jivesoftware.smack.AbstractXMPPConnection.connect (AbstractXMPPConnection.java:530)
at org.jivesoftware.smack.ReconnectionManager$2.run (ReconnectionManager.java:282)
at java.lang.Thread.run (Thread.java:784)
