Coverity: fix issues reported in QJsonModel

- Also reported upstream, in the MIT-licensed version of same.

This is a re-hash of b348a45834,
adding memory-leak prevention when load() is called.

src/libcalamaresui/utils/qjsonitem.cpp

@@ -26,11 +26,9 @@
 #include "qjsonitem.h"
 
 QJsonTreeItem::QJsonTreeItem(QJsonTreeItem *parent)
+    : mParent( parent )
+    , mType( QJsonValue::Type::Null )
 {
-
-    mParent = parent;
-
-
 }
 
 QJsonTreeItem::~QJsonTreeItem()

src/libcalamaresui/utils/qjsonmodel.cpp

@@ -33,14 +33,19 @@
 
 QJsonModel::QJsonModel(QObject *parent) :
     QAbstractItemModel(parent)
+    , mRootItem( new QJsonTreeItem )
 {
-    mRootItem = new QJsonTreeItem;
     mHeaders.append("key");
     mHeaders.append("value");
 
 
 }
 
+QJsonModel::~QJsonModel()
+{
+    delete mRootItem;
+}
+
 bool QJsonModel::load(const QString &fileName)
 {
     QFile file(fileName);
@@ -66,6 +71,7 @@ bool QJsonModel::loadJson(const QByteArray &json)
     if (!mDocument.isNull())
     {
         beginResetModel();
+        delete mRootItem;
         if (mDocument.isArray()) {
             mRootItem = QJsonTreeItem::load(QJsonValue(mDocument.array()));
         } else {