fix a segfault when accessing a repeated field with an improper unico…

…de string member

src/fastpb/template/module.jinjacc

@@ -168,17 +168,17 @@ namespace {
           new google::protobuf::io::StringOutputStream(&result);
       google::protobuf::io::CodedOutputStream* outputStream =
           new google::protobuf::io::CodedOutputStream(output);
-    
+
       PyObject *sequence = PySequence_Fast(values, "The values to serialize must be a sequence.");
       for (Py_ssize_t i = 0, len = PySequence_Length(sequence); i < len; ++i) {
           {{ message.name }} *value = ({{ message.name }} *)PySequence_Fast_GET_ITEM(sequence, i);
-        
+
           Py_BEGIN_ALLOW_THREADS
           outputStream->WriteVarint32(value->protobuf->ByteSize());
           value->protobuf->SerializeToCodedStream(outputStream);
           Py_END_ALLOW_THREADS
       }
-    
+
       Py_XDECREF(sequence);
       delete outputStream;
       delete output;
@@ -297,6 +297,9 @@ namespace {
             PyObject *value =
                 fastpb_convert{{ member.type if member.type != TYPE.MESSAGE else message.name + member.name }}(
                     self->protobuf->{{ member.name.lower() }}(i));
+            if (!value) {
+              return NULL;
+            }
             PyTuple_SetItem(tuple, i, value);
           }
           return tuple;