QSerialPortInfo FreeBSD: assert the preconditions in a helper function

The fact that previousOid should not be empty is a precondition for
the nextOid() helper function. This is currently checked at the call
sites. Add a Q_ASSERT() to guarantee that a future refactoring does
not skip the condition.

Task-number: QTBUG-144899
Change-Id: I2133e5bb312d258ec311ebe5d36f8bec03cbc355
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>