Make Entry::m_tmpHistoryItem a QScopedPointer (#2524)

* Make m_tmpHistoryItem a QScopedPointer

Most of the time, `m_tmpHistoryItem` should be null by the time an
`Entry` is destroyed. However, if a caller ever calls `beginUpdate()`
without later calling `endUpdate()` -- perhaps because an exception was
throw in the meantime -- it may not be null. This change avoids a memory 
leak in that case.

Found via https://lgtm.com/projects/g/keepassxreboot/keepassxc/alerts

src/core/Entry.h

@@ -249,7 +249,7 @@ private:
     QPointer<CustomData> m_customData;
     QList<Entry*> m_history; // Items sorted from oldest to newest
 
-    Entry* m_tmpHistoryItem;
+    QScopedPointer<Entry> m_tmpHistoryItem;
     bool m_modifiedSinceBegin;
     QPointer<Group> m_group;
     bool m_updateTimeinfo;