Handle Panel_getSelected() returning NULL

Found by compiling with LTO:

    ColumnsPanel.c: In function ‘ColumnsPanel_eventHandler’:
    ColumnsPanel.c:46:59: error: potential null pointer dereference [-Werror=null-dereference]
       46 |             ((ListItem*)Panel_getSelected(super))->moving = this->moving;
          |                                                           ^
    AvailableColumnsPanel.c: In function ‘AvailableColumnsPanel_eventHandler’:
    AvailableColumnsPanel.c:31:8: error: potential null pointer dereference [-Werror=null-dereference]
       31 |    int key = ((ListItem*) Panel_getSelected(super))->key;
          |        ^
    AvailableMetersPanel.c: In function ‘AvailableMetersPanel_eventHandler’:
    AvailableMetersPanel.c:40:24: error: potential null pointer dereference [-Werror=null-dereference]
       40 |    int param = selected->key & 0xff;
          |                        ^
    linux/IOPriorityPanel.c: In function ‘IOPriorityPanel_getIOPriority’:
    linux/IOPriorityPanel.c:37:11: error: potential null pointer dereference [-Werror=null-dereference]
       37 |    return (IOPriority) ( ((ListItem*) Panel_getSelected(this))->key );
          |           ^

0 files changed, 0 insertions, 0 deletions