remote: don't write too much when dealing with multivars

remote: don't write too much when dealing with multivars

We used to move `data_start` forward, which is wrong as that needs to
point to the beginning of the buffer in order to perform size
calculations.

Introduce a `write_start` variable which indicates where we should start
writing from, which is what the `data_start` was being wrongly reused to
be.