avoid floating point exception when _EM_OVERFLOW is enabled (#2303)

imgui.cpp

@@ -3729,7 +3729,7 @@ void ImGui::EndFrame()
     IM_ASSERT(g.FrameScopeActive && "Forgot to call ImGui::NewFrame()?");
 
     // Notify OS when our Input Method Editor cursor has moved (e.g. CJK inputs using Microsoft IME)
-    if (g.IO.ImeSetInputScreenPosFn && ImLengthSqr(g.PlatformImeLastPos - g.PlatformImePos) > 0.0001f)
+    if (g.IO.ImeSetInputScreenPosFn && (g.PlatformImeLastPos.x == FLT_MAX || ImLengthSqr(g.PlatformImeLastPos - g.PlatformImePos) > 0.0001f))
     {
         g.IO.ImeSetInputScreenPosFn((int)g.PlatformImePos.x, (int)g.PlatformImePos.y);
         g.PlatformImeLastPos = g.PlatformImePos;