Reduce the number of places we modify 'offset' so that
 op_seek_helper() can always skip seeks to the current offset.
The checks we were doing before already covered all the places
 where this was useful in the normal case, but this lets us
 centralize that logic.

This commit also includes a few minor follow-ups to 9b57b0c2:
* Use a smaller type for ret_size and initialize it.
* Verify 'end' is at least as large as data we've already read.