Fixed GCC zealous warnings (#1623)

imgui.cpp

@@ -681,6 +681,7 @@
 #pragma GCC diagnostic ignored "-Wconversion"               // warning: conversion to 'xxxx' from 'xxxx' may alter its value
 #pragma GCC diagnostic ignored "-Wcast-qual"                // warning: cast from type 'xxxx' to type 'xxxx' casts away qualifiers
 #pragma GCC diagnostic ignored "-Wformat-nonliteral"        // warning: format not a string literal, format string not checked
+#pragma GCC diagnostic ignored "-Wstrict-overflow"          // warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false
 #endif
 
 // Enforce cdecl calling convention for functions called by the standard library, in case compilation settings changed the default to e.g. __vectorcall
@@ -4876,6 +4877,7 @@ static ImGuiWindow* GetFrontMostModalRootWindow()
 
 static void ClosePopupToLevel(int remaining)
 {
+    IM_ASSERT(remaining >= 0);
     ImGuiContext& g = *GImGui;
     ImGuiWindow* focus_window = (remaining > 0) ? g.OpenPopupStack[remaining-1].Window : g.OpenPopupStack[0].ParentWindow;
     if (g.NavLayer == 0)
@@ -11215,7 +11217,7 @@ bool ImGui::BeginMenu(const char* label, bool enabled)
     if (!enabled) // explicitly close if an open menu becomes disabled, facilitate users code a lot in pattern such as 'if (BeginMenu("options", has_object)) { ..use object.. }'
         want_close = true;
     if (want_close && IsPopupOpen(id))
-        ClosePopupToLevel(GImGui->CurrentPopupStack.Size);
+        ClosePopupToLevel(g.CurrentPopupStack.Size);
 
     if (!menu_is_open && want_open && g.OpenPopupStack.Size > g.CurrentPopupStack.Size)
     {