Some calls to btr_pcur_close() are unnecessary

This came up while I was working on MDEV-13542. The function btr_pcur_close() is being invoked on local variables even when no cleanup needs to be done. Outside SPATIAL INDEX, the only needed cleanup might be freeing the buffer for old_rec, but only if btr_pcur_store_position() might have been invoked on the object. For spatial index, there is some r-tree path information that needs to be freed as well.