Small fix rbart out_of_group random effect

[brunotancredi]

Hi,

I've been working with the rbart_vi function and encountered an issue when making predictions with out-of-sample groups in the test. After reviewing the code, I've identified that the error is due to a mismatch in dimensions within the rep. I believe that this small change is the correct fix. I've run the code with this small variation, and it works.