IMPALA-964: Full outer join on values() followed by group by hits a
preconditions check

This commit fixes IMPALA-964 where full outer join between two inline
views followed by a group by (e.g. select 1 FROM (VALUES(1 x, 1 y)) a
FULL OUTER JOIN (VALUES(1 x, 1 y)) b ON (a.x = b.y) GROUP BY a.x;)
hits a preconditions check. This check evaluates if the numNodes
(number of nodes for the purpose of resource estimation) variable
is greater or equal to zero and is triggered when we try to compute
the resource estimates (number of distinct values) of a plan fragment.

The following changes are included in this commit:
1. Modified the getNumDistinctValues function in PlanFragment class to
consider the special case where the numNodes of a plan fragment is -1.
2. Added a test case in QueryTest/joins.test.

Change-Id: I2962ed5079e174d0e76ad990ab84e1fb1a4607ef
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2466
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins