[Qt] small changes to sendcoinsdialog

- add newly added variables to the constructor init
- move an already existing bool also to constructor init
- add a missing NULL pointer check for clientModel (and add a comment
  above that part of the code)
It was added by @cozz and is used in this line:
``connect(clientModel, SIGNAL(numBlocksChanged(int)), this, SLOT(updateSmartFeeLabel()));``

That comment can perhaps just be removed... I just found it good to know that in that setModel() call we can be sure that setClientModel() was called (in current code).