Fix more GCC 8.1 warnings  1   master

Fix 3 warnings reported by GCC 8.1 of the following type

```src/client/gameui.cpp:191:43: warning: « void* memset(void*, int, size_t) » effacement d'un objet du type non trivial « struct GameUI::Flags »; use assignment or value-initialization instead [-Wclass-memaccess]
  memset(&m_flags, 0, sizeof(GameUI::Flags));
```

src/gui/guiTable.cpp

@@ -334,7 +334,6 @@ void GUITable::setTable(const TableOptions &options,
 
 		// Make template for new cells
 		Cell newcell;
-		memset(&newcell, 0, sizeof newcell);
 		newcell.content_type = columntype;
 		newcell.tooltip_index = tooltip_index;
 		newcell.reported_column = j+1;