DeckEditor: Initialize the modified flag (#5743)

C++ does not require compilers to zero-initialize value types, so
depending on the platform (here: Linux), the deck editor starts up with
an uninitialized value in the `modified` flag, which is usually not
zero.

cockatrice/src/client/tabs/abstract_tab_deck_editor.h

@@ -147,7 +147,7 @@ protected:
     QAction *aCardInfoDockVisible, *aCardInfoDockFloating, *aDeckDockVisible, *aDeckDockFloating;
     QAction *aFilterDockVisible, *aFilterDockFloating, *aPrintingSelectorDockVisible, *aPrintingSelectorDockFloating;
 
-    bool modified;
+    bool modified = false;
 };
 
 #endif // TAB_GENERIC_DECK_EDITOR_H