Fix incorrect bitmap copying and offset shifting in GenerateUnsafeRowJoiner

The following query returns an incorrect answer:

set spark.sql.autoBroadcastJoinThreshold=-1;

create table a as select * from values 1;
create table b as select * from values 2;

SELECT
t3.col1,
t1.col1
FROM a t1
CROSS JOIN b t2
CROSS JOIN b t3

This should return the row 2, 1 but instead it returns null, 1. If you permute the order of the columns in the select statement or the order of the joins then it returns a valid answer (i.e. one without incorrect NULLs).

This turns out to be due to two longstanding bugs in GenerateUnsafeRowJoiner, which I'll describe in more detail in my PR.