pgsql: Avoid memcpy() with a NULL source pointer and count == 0

Avoid memcpy() with a NULL source pointer and count == 0

When memcpy() is called on a pointer, the compiler is entitled to assume
that the pointer is not null, which can lead to optimizing nearby code
in potentially undesirable ways. We still want such optimizations
(gcc's -fdelete-null-pointer-checks) in cases where they're valid.

Related: commit 13bba02271dc.

Backpatch to pg11, where this particular instance appeared.

Reported-by: Ranier Vilela <ranier(dot)vf(at)gmail(dot)com>
Reported-by: Zhihong Yu <zyu(at)yugabyte(dot)com>
Discussion: https://postgr.es/m/CAEudQApUndmQkr5fLrCKXQ7+ib44i7S+Kk93pyVThS85PnG3bQ@mail.gmail.com
Discussion: https://postgr.es/m/CALNJ-vSdhwSM5f4tnNn1cdLHvXMVe_S+V3nR5GwNrmCPNB2VtQ@mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5a1d1b9540a4f5bf4ee6761a17b21ad7c0012b49

Modified Files
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src/backend/commands/indexcmds.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)