BUG/MEDIUM: sample: Fix adjusting size in field converter

BUG/MEDIUM: sample: Fix adjusting size in field converter

Adjust the size of the sample buffer before we change the "area"
pointer. The change in size is calculated as the difference between the
original pointer and the new start pointer. But since the
`smp->data.u.str.area` assignment results in `smp->data.u.str.area` and
`start` being the same pointer, we always ended up substracting zero.
This changes it to change the size by the actual amount it changed.

I'm not entirely sure what the impact of this is, but the previous code
seemed wrong.

[wt: from what I can see the only harmful case is when the output is
 converted to a stick-table key, it could result in zeroing past the
 end of the buffer; other cases do not touch beyond ->data]