IMPALA-901: Fix grouping with NULLs when codegen is disabled

The standard implementation of HashTable::Equals() did not correctly
check the NULL bit when the argument row did not evaluate to NULL for a
given probe expr. In the rare circumstance that this gave rise to a
false positive (more on that below), two rows with different grouping
values would be considered equal, and one would be excluded from the
final aggregation output.

HashTable::EvalRow() fills an expression value buffer with the values of
either probe or build exprs evaluated for the argument row. These cached
values are used to determine row equality in Equals(). In order to avoid
a lot of false collisions, an 'unlikely' value is written to that buffer
for NULL values, chosen to be HashUtil::FNV_SEED. So without correct
NULL-bit checking in Equals(), two single-slot rows are considered to be
equal if one of them has NULL for its slot, and the other has a value
equal to HashUtil::FNV_SEED truncated to the size of the slot.

For tinyint columns, this value is -59. As it happens, our random
generator happened to create a table with one tinyint column and which
contained NULL and -59 as values. In order to trigger this bug, the rows
must also have been written to disk in order such that the scanners
returned -59 *first*, and then NULL to the aggregation node; the bug is
not symmetric and works in the opposite case.

Change-Id: I17d43eaeee62b2ac01b67dd599bc4346b012a074
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2130
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins