Here we're allocating memory for a new buffer, then attempting to call bson_buffer_set_max_size before performing a null check. At this point in this code path, the null check isn't necessary and should be done before we try to set the buffer's max_size.

bson_buffer_t buffer = bson_buffer_new();
bson_buffer_set_max_size(buffer, FIX2INT(max_size));
if (buffer == NULL) {
    rb_raise(rb_eNoMemError, "failed to allocate memory in buffer.c");
}

Link:
https://github.com/mongodb/mongo-ruby-driver/blob/master/ext/cbson/cbson.c#L618-L622