rows_where() raises OperationalError when offset is given without limit #816

Passing offset to rows_where() (or pks_and_rows_where()) without also passing limit generates invalid SQL and raises OperationalError.

Repro:

import sqlite_utils

db = sqlite_utils.Database(":memory:")
db["t"].insert_all([{"id": i} for i in range(5)])
list(db["t"].rows_where(offset=2))
# OperationalError: near "2": syntax error

Root cause: In db.py rows_where() appends OFFSET N to the SQL only when limit is not None is False, but SQLite requires LIMIT to appear before OFFSET. The generated SQL is SELECT * FROM "t" OFFSET 2, which is invalid syntax.

The same issue affects search_sql() when offset is given without limit.

Fix: When offset is set but limit is not, emit LIMIT -1 before the OFFSET clause. SQLite treats a negative limit as "no upper bound", so LIMIT -1 OFFSET N correctly returns all rows starting from position N.