Throw NotConnectedException on login()

if connect() was not previously called. Previously calling login() with
arguments would not check for the preconditions.

The check to throw needs to be performed in AbstractXMPPConnection
before every 'abstract login(Non)Anonymously()' call. That's the two
lines that check the preconditions are duplicated.

Also fix NPE in
XMPPTCPConnection.throwNotConnectedExceptionIfAppropriate() when
packetWrite is null (i.e. if the connection was never connected before).

smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java

@@ -393,9 +393,9 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
      * @throws IOException 
      */
     public void login() throws XMPPException, SmackException, IOException {
-        throwNotConnectedExceptionIfAppropriate();
-        throwAlreadyLoggedInExceptionIfAppropriate();
         if (isAnonymous()) {
+            throwNotConnectedExceptionIfAppropriate();
+            throwAlreadyLoggedInExceptionIfAppropriate();
             loginAnonymously();
         } else {
             // The previously used username, password and resource take over precedence over the
@@ -440,6 +440,8 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
         if (!config.allowNullOrEmptyUsername && StringUtils.isNullOrEmpty(username)) {
             throw new IllegalArgumentException("Username must not be null or empty");
         }
+        throwNotConnectedExceptionIfAppropriate();
+        throwAlreadyLoggedInExceptionIfAppropriate();
         usedUsername = username;
         usedPassword = password;
         usedResource = resource;

smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java

@@ -306,6 +306,9 @@ public class XMPPTCPConnection extends AbstractXMPPConnection {
 
     @Override
     protected void throwNotConnectedExceptionIfAppropriate() throws NotConnectedException {
+        if (packetWriter == null) {
+            throw new NotConnectedException();
+        }
         packetWriter.throwNotConnectedExceptionIfDoneAndResumptionNotPossible();
     }