ast.go: added recover from panic in ast.Eval()

ast.go

@@ -5,6 +5,7 @@
 package expr
 
 import (
+	"errors"
 	"strings"
 
 	"git.portale-stac.it/go-pkg/expr/kern"
@@ -106,6 +107,17 @@ func (ast *ast) Finish() {
 }
 
 func (ast *ast) Eval(ctx kern.ExprContext) (result any, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			if errVal, ok := r.(error); ok {
+				err = errVal
+			}
+		}
+		if err == nil {
+			err = errors.New("unexpected error while evaluating the expression")
+		}
+	}()
+
 	ast.Finish()
 
 	if ast.root != nil {