[libqofono] Fix crash in QOfonoCallForwarding::isValid()

We shouldn't crash if d_ptr->callForward is NULL
(e.g. modem path was never set)
Merge pull request #61 from monich/qofonocallforwarding

Fix crash in QOfonoCallForwarding::isValid()
[libqofono] Added QOfonoObject and 'valid' property

QOfonoObject encapsulates asynchronous properties query and property change
signal handling. This is a common pattern for many ofono objects.

The 'valid' property on the affected objects becomes true after the initial
GetProperties call completes. That way the user of the object can find out
that the object has finished initializing and its properties actually mean
anything.
Merge pull request #64 from monich/qofonoobject

Added QOfonoObject class and 'valid' property
Bump version to 0.64