Tabs: Fix initial tab state. Amend 3e6f948( (#5572)

Somehow this only triggers a failing test in docking branch.

imgui_internal.h

@@ -2220,7 +2220,7 @@ struct ImGuiTabItem
     ImS16               IndexDuringLayout;      // Index only used during TabBarLayout()
     bool                WantClose;              // Marked as closed by SetTabItemClosed()
 
-    ImGuiTabItem()      { memset(this, 0, sizeof(*this)); LastFrameVisible = LastFrameSelected = -1; NameOffset = -1; BeginOrder = IndexDuringLayout = -1; }
+    ImGuiTabItem()      { memset(this, 0, sizeof(*this)); LastFrameVisible = LastFrameSelected = -1; RequestedWidth = -1.0f; NameOffset = -1; BeginOrder = IndexDuringLayout = -1; }
 };
 
 // Storage for a tab bar (sizeof() 152 bytes)