Fix atEnd() and canReadLine() methods
Fix atEnd() and canReadLine() methods

Previously, these methods are used the size of the readBuffer in
internal implementation. This is incorrect because the buffer size
does not always contain the actual information (for example on Win32
I/O implementation).

Now, instead of getting the direct size of readBuffer, used private
method bytesAvailable().

Change-Id: I36b0db1be169c753cc62d6650e498ee8bf388807
Reviewed-by: Sergey Belyashov <8871b4c2d635fca691211507eadbac3d0bc2695a@gmail.com>
Reviewed-by: Denis Shienkov <6a9c747bd2ba446682324cf340d3b86f30cf4d55@gmail.com>

kobolabs/qtserialport,kobolabs/qtserialport,RoadWorksSoftware/QtSerialPort-OS2,RoadWorksSoftware/QtSerialPort-OS2