When we create an ImmutableRow::from_value(), we are always adding a
null padding at the end. We didn't notice this before, because a SQLite
file with an extra column is as valid as any. But that column of course
should not be there.
I traced this to column_count(), which is off by one. My understanding
is that we should be returning based on serial_types, not offset.
Closes#2862
efc105d99e