Fix uninitialized m_isNew causing asterisk on all DRC Rule Editor tree items

The default constructor of RULE_EDITOR_DATA_BASE left m_isNew
uninitialized. On Linux, the garbage memory value was assigned
causing IsNew() to return true for every rule and appending
astersix to all tree items.

Fix by adding a default member initializer: m_isNew = false.

include/dialogs/rule_editor_data_base.h

@@ -141,7 +141,7 @@ private:
     std::optional<int> m_parentId; // Optional parent ID of the rule.
     wxString           m_ruleName;
     wxString           m_comment;
-    bool               m_isNew; /**< Flag indicating if the user is creating a new rule (true) 
+    bool               m_isNew = false; /**< Flag indicating if the user is creating a new rule (true) 
                                               or editing an existing rule (false). */
 };