terminalpanel.cpp: Set m_konsolePart to nullptr during exit signal
If Dolphin terminal is open, and one types exit there and then closes Dolphin, Dolphin crashes due to a dangling pointer.
Konsole KPart is deleting itself when Konsole session exits, but Dolphin tracks it as a child of the Terminal Panel. The Terminal Panel doesn't get destroyed when Konsole KPart does, and it tries to double free during children clean up. (Thanks @sitter !)
Setting m_konsolePart to nullptr during exit signal fixes this, since there is already check in place for that in the destructor.
Edited by Akseli Lahtinen