Fix bugs in receiveSingleIteration and optimize sendMessageVector

[AniruddhaKanhere]

This PR fixes a bug in receiveSingleIteration function which caused existing data not to be processed till new data was received on the transport interface. The bug is fixed by adding an additional check in the MQTTNoDataAvailable case as below:

-   else if( recvBytes == 0 )
+   else if( ( recvBytes == 0 ) && ( pContext->index == 0 ) )
    {
        /* No more bytes available since the last read and neither is anything in
         * the buffer. */
        status = MQTTNoDataAvailable;
    }

This PR also fixes the issue in the send function where sending with 0 timeout failed as the time was checked before an attempt to send. The fix moves the time check after the send call thus allowing at least one send even with 0 timeout set.