fix: check obj type in protobufBinding (gin-gonic#2851)

* fix: check obj type in protobufBinding

* fix: UnitTest for invalid proto obj

binding/binding_test.go

@@ -1339,6 +1339,13 @@ func testProtoBodyBindingFail(t *testing.T, b Binding, name, path, badPath, body
 	err := b.Bind(req, &obj)
 	assert.Error(t, err)
 
+	invalid_obj := FooStruct{}
+	req.Body = ioutil.NopCloser(strings.NewReader(`{"msg":"hello"}`))
+	req.Header.Add("Content-Type", MIMEPROTOBUF)
+	err = b.Bind(req, &invalid_obj)
+	assert.Error(t, err)
+	assert.Equal(t, err.Error(), "obj is not ProtoMessage")
+
 	obj = protoexample.Test{}
 	req = requestWithBody("POST", badPath, badBody)
 	req.Header.Add("Content-Type", MIMEPROTOBUF)

binding/protobuf.go

@@ -5,6 +5,7 @@
 package binding
 
 import (
+	"errors"
 	"io/ioutil"
 	"net/http"
 
@@ -26,7 +27,11 @@ func (b protobufBinding) Bind(req *http.Request, obj interface{}) error {
 }
 
 func (protobufBinding) BindBody(body []byte, obj interface{}) error {
-	if err := proto.Unmarshal(body, obj.(proto.Message)); err != nil {
+	msg, ok := obj.(proto.Message)
+	if !ok {
+		return errors.New("obj is not ProtoMessage")
+	}
+	if err := proto.Unmarshal(body, msg); err != nil {
 		return err
 	}
 	// Here it's same to return validate(obj), but util now we can't add