pgsql: Clean up assorted failures under clang's -fsanitize=undefined ch

Clean up assorted failures under clang's -fsanitize=undefined checks.

Most of these are cases where we could call memcpy() or other libc
functions with a NULL pointer and a zero count, which is forbidden
by POSIX even though every production version of libc allows it.
We've fixed such things before in a piecemeal way, but apparently
never made an effort to try to get them all. I don't claim that
this patch does so either, but it gets every failure I observe in
check-world, using clang 12.0.1 on current RHEL8.

numeric.c has a different issue that the sanitizer doesn't like:
"ln(-1.0)" will compute log10(0) and then try to assign the
resulting -Inf to an integer variable. We don't actually use the
result in such a case, so there's no live bug.

Back-patch to all supported branches, with the idea that we might
start running a buildfarm member that tests this case. This includes
back-patching c1132aae3 (Check the size in COPY_POINTER_FIELD),
which previously silenced some of these issues in copyfuncs.c.

Discussion: https://postgr.es/m/CALNJ-vT9r0DSsAOw9OXVJFxLENoVS_68kJ5x0p44atoYH+H4dg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/46ab07ffda9d6c8e63360ded2d4568aa160a7700

Modified Files
--------------
contrib/pgcrypto/px.c | 2 +-
src/backend/access/heap/heapam.c | 2 +-
src/backend/access/heap/heapam_visibility.c | 4 ++--
src/backend/access/transam/clog.c | 5 +++--
src/backend/access/transam/xact.c | 5 +++--
src/backend/storage/ipc/shm_mq.c | 7 +++++--
src/backend/utils/adt/numeric.c | 8 ++++++++
src/backend/utils/time/snapmgr.c | 10 ++++++----
src/fe_utils/print.c | 3 ++-
9 files changed, 31 insertions(+), 15 deletions(-)