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

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