Fix harmless float calculation overflow. (#2813)

imgui.cpp

@@ -2711,6 +2711,8 @@ ImGuiWindow::ImGuiWindow(ImGuiContext* context, const char* name)
     SetWindowPosAllowFlags = SetWindowSizeAllowFlags = SetWindowCollapsedAllowFlags = ImGuiCond_Always | ImGuiCond_Once | ImGuiCond_FirstUseEver | ImGuiCond_Appearing;
     SetWindowPosVal = SetWindowPosPivot = ImVec2(FLT_MAX, FLT_MAX);
 
+    InnerRect = ImRect(0.0f, 0.0f, 0.0f, 0.0f); // Clear so the InnerRect.GetSize() code in Begin() doesn't lead to overflow even if the result isn't used.
+
     LastFrameActive = -1;
     LastTimeActive = -1.0f;
     ItemWidthDefault = 0.0f;