IMPALA-770: Fix crash in aggregation node with zero-width tuple

The select exprs of an inline view may not always be materialised, yet
the output tuple itself may be. This patch fixes a crash in this
situation in the backend aggregation node which assumed its output tuple
would always have at least one materialised slot.

The cause was a couple of too-conservative DCHECKs that failed if the
tuple was NULL. In fact, the code was robust to this possibility without
the checks, so this bug didn't affect release builds of Impala.

Change-Id: If0b90809d30fcd196f55197953392452d1ac9c4f
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1431
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins