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
------
master

Details
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http://git.postgresql.org/pg/commitdiff/65fd91333e45114c5d9a07d3d4f6a4786df48768

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