unmarshal: fix when second doc is invalid

loader.go

@@ -129,8 +129,8 @@ func UnmarshalJSON(r io.Reader) (any, error) {
 	if err := decoder.Decode(&doc); err != nil {
 		return nil, err
 	}
-	if t, _ := decoder.Token(); t != nil {
-		return nil, fmt.Errorf("invalid character %v after top-level value", t)
+	if _, err := decoder.Token(); err == nil || err != io.EOF {
+		return nil, fmt.Errorf("invalid character after top-level value")
 	}
 	return doc, nil
 }

loader_test.go

@@ -0,0 +1,28 @@
+package jsonschema
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestUnmarshalJSON(t *testing.T) {
+	tests := []struct {
+		input string
+		valid bool
+	}{
+		{"{", false},
+		{"{}", true},
+		{"{}A", false},
+		{"{}{}", false},
+	}
+
+	for _, test := range tests {
+		_, err := UnmarshalJSON(strings.NewReader(test.input))
+		if valid := err == nil; valid != test.valid {
+			if err != nil {
+				t.Log(err)
+				t.Errorf("UnmarshalJSON(%q) valid: got %v, want %v", test.input, valid, test.valid)
+			}
+		}
+	}
+}