sql: refactor VDBE opcode OP_OffsetLimit

OP_OffsetLimit instruction calculates sum of OFFSET and LIMIT values
when they are present. This sum serves as a counter of entries to be
inserted to temporary space during VDBE execution. Consider query like:

SELECT * FROM t ORDER BY x LIMIT 5 OFFSET 2;

To perform ordering alongside with applying limit and offset
restrictions, first 7 (5 + 2) entries are inserted into temporary space.
They are sorted and then first two tuples are skipped according to offset
clause. The rest tuples from temporary space get to result set.

When sum of LIMIT and OFFSET values is big enough to cause integer
overflow, we can't apply this approach. Previously, counter was simply
set to -1 which means that all entries from base table will be transferred
to ephemeral space. As a result, LIMIT clause was ignored and the result
of query would be incorrect. Motivation for this obviously wrong step was
that to perform query with such huge limit and offset values too many time
is required (like years). What is more, ephemeral spaces support
auto-generated IDs in the range up to 2^32, so there's even no opportunity
to process such queries in theory. Nevertheless, to make code cleaner
let's fix this tricky place and just raise an overflow error if result
of addition exceeds integer range.

This patch fixes obsolete comments saying that in case of overflow
execution won't stop; now limit and offset counter are always >= 0, so
removed redundant branching.