Fix rows_where() crash when offset is given without limit

SQLite requires LIMIT before OFFSET; omitting it raised OperationalError.
When offset is set and limit is not, emit LIMIT -1 first so the SQL is valid.
Same fix applied to search_sql(). Adds regression test.

sqlite_utils/db.py

@@ -2003,6 +2003,8 @@ class Queryable:
         if limit is not None:
             sql += f" limit {limit}"
         if offset is not None:
+            if limit is None:
+                sql += " limit -1"
             sql += f" offset {offset}"
         cursor = self.db.execute(sql, where_args or [])
         columns = dedupe_keys(c[0] for c in cursor.description)
@@ -3594,6 +3596,8 @@ class Table(Queryable):
         if limit is not None:
             limit_offset += f" limit {limit}"
         if offset is not None:
+            if limit is None:
+                limit_offset += " limit -1"
             limit_offset += f" offset {offset}"
         return sql.format(
             dbtable=quote_identifier(self.name),