Field index bug with AS OF query and secondary index

[fulghum]

When using AS OF queries with a secondary index, Dolt returns a unable to find field error in some cases:

-- Create a table with a secondary index in the first Dolt commit
create table t (pk int primary key, c1 varchar(255), key c1_idx(c1));
insert into t values (1, 'one');
call dolt_commit('-Am', 'adding table t with index on main');
SET @commit1 = hashof('HEAD');

-- Trying to query with `AS OF` when using the secondary index causes an error
SELECT c1 from t as of @commit1 where c1 > 'o';
unable to find field with index 1 in row of 1 columns

-- Not using the second index works correctly
SELECT c1 from t as of @commit1;
+-----+
| c1  |
+-----+
| one |
+-----+