pgsql: Fix an Assert that turns out to be reachable after all.

Fix an Assert that turns out to be reachable after all.

estimate_num_groups() gets unhappy with
create table empty();
select * from empty except select * from empty e2;
I can't see any actual use-case for such a query (and the table is illegal
per SQL spec), but it seems like a good idea that it not cause an assert
failure.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/0e20abdb47e3499353ac4fd1b2d21e82ec6b1461

Modified Files
--------------
src/backend/utils/adt/selfuncs.c | 9 +++++++--
1 files changed, 7 insertions(+), 2 deletions(-)