Fixed segfault when reading Caffe model

modules/dnn/src/caffe/caffe_importer.cpp

@@ -125,6 +125,7 @@ public:
         {
             const google::protobuf::UnknownField& field = unknownFields.field(i);
             CV_Assert(field.type() == google::protobuf::UnknownField::TYPE_GROUP);
+            CV_CheckGE(field.group().field_count(), 2, "UnknownField should have at least 2 items: name and value");
             std::string fieldName = field.group().field(0).length_delimited();
             std::string fieldValue = field.group().field(1).length_delimited();
             params.set(fieldName, fieldValue);