pgsql: Test ECPG decadd(), decdiv(), decmul(), and decsub() for risnull

Test ECPG decadd(), decdiv(), decmul(), and decsub() for risnull() input.

Since commit 757fb0e5a9a61ac8d3a67e334faeea6dc0084b3f, these
Informix-compat functions return 0 without changing the output
parameter. Initialize the output parameter before the test call, making
that obvious. Before this, the expected test output has been depending
on freed stack memory. "gcc -ftrivial-auto-var-init=pattern" revealed
that. Back-patch to v13 (all supported versions).

Discussion: https://postgr.es/m/20250106192748.cf.nmisch@google.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7819a25cd101b574f5422edb00fe3376fbb646da

Modified Files
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.../ecpg/test/compat_informix/dec_test.pgc | 10 +
.../ecpg/test/expected/compat_informix-dec_test.c | 10 +
.../test/expected/compat_informix-dec_test.stdout | 232 ++++++++++-----------
3 files changed, 136 insertions(+), 116 deletions(-)