Fix internal buffer corruption in QBluetoothSocket (BlueZ)

When _q_readNotify() is called and no data is available (e.g. in case
of timeout), then the internal buffer grows indefinitely, causing a
one time corruption of the following good data.

Change-Id: I75929382db73953bbccdd86b9ae248329df879fa
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>