fix: add panic recovery and boundary checks for Math function

internal/captcha/handler.go

@@ -282,7 +282,14 @@ func (h *Handler) loadCharset() ([]string, error) {
 
 const mathChars = "0123456789+-*/÷×=?"
 
-func (h *Handler) Math(imageBase64 string) (string, error) {
+func (h *Handler) Math(imageBase64 string) (result string, err error) {
+	// 捕获 panic
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("Math panic: %v", r)
+		}
+	}()
+
 	sess, ok := onnx.GetSession("math")
 	if !ok {
 		return "", fmt.Errorf("Math 模型未加载")
@@ -315,7 +322,7 @@ func (h *Handler) Math(imageBase64 string) (string, error) {
 		return "", fmt.Errorf("无法识别表达式")
 	}
 
-	result, err := evalMathExpression(expr)
+	result, err = evalMathExpression(expr)
 	if err != nil {
 		return "", err
 	}
@@ -330,11 +337,23 @@ func preprocessMath(img image.Image) ([]float32, error) {
 	// 计算缩放比例
 	bounds := img.Bounds()
 	srcW, srcH := bounds.Dx(), bounds.Dy()
+	if srcW <= 0 || srcH <= 0 {
+		return nil, fmt.Errorf("无效的图片尺寸")
+	}
+	
 	ratio := minFloat(float64(targetW)/float64(srcW), float64(targetH)/float64(srcH))
 	
 	newW := int(float64(srcW) * ratio)
 	newH := int(float64(srcH) * ratio)
 	
+	// 确保至少 1 像素
+	if newW <= 0 {
+		newW = 1
+	}
+	if newH <= 0 {
+		newH = 1
+	}
+	
 	// 缩放图片
 	resized := imaging.Resize(img, newW, newH, imaging.Lanczos)