pgsql: Fix old oversight in const-simplification of COALESCE() expressi

Fix old oversight in const-simplification of COALESCE() expressions.

Once we have found a non-null constant argument, there is no need to
examine additional arguments of the COALESCE.  The previous coding got it
right only if the constant was in the first argument position; otherwise
it tried to simplify following arguments too, leading to unexpected
behavior like this:

regression=# select coalesce(f1, 42, 1/0) from int4_tbl;
ERROR:  division by zero

It's a minor corner case, but a bug is a bug, so back-patch all the way.

Branch
------
REL8_3_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=7b3c4a5d43e9529984ddbad501014f44d15bb34c

Modified Files
--------------
src/backend/optimizer/util/clauses.c |    6 +++++-
1 files changed, 5 insertions(+), 1 deletions(-)