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 13bba022.

Backpatch to pg11, where this particular instance appeared.
Reported-by: Ranier Vilela <ranier.vf@gmail.com>
Reported-by: Zhihong Yu <zyu@yugabyte.com>
Discussion: https://postgr.es/m/CAEudQApUndmQkr5fLrCKXQ7+ib44i7S+Kk93pyVThS85PnG3bQ@mail.gmail.com
Discussion: https://postgr.es/m/CALNJ-vSdhwSM5f4tnNn1cdLHvXMVe_S+V3nR5GwNrmCPNB2VtQ@mail.gmail.com

src/backend/commands/indexcmds.c

@@ -1163,15 +1163,17 @@ DefineIndex(Oid relationId,
 
 	if (partitioned)
 	{
+		PartitionDesc partdesc;
+
 		/*
 		 * Unless caller specified to skip this step (via ONLY), process each
 		 * partition to make sure they all contain a corresponding index.
 		 *
 		 * If we're called internally (no stmt->relation), recurse always.
 		 */
-		if (!stmt->relation || stmt->relation->inh)
+		partdesc = RelationGetPartitionDesc(rel);
+		if ((!stmt->relation || stmt->relation->inh) && partdesc->nparts > 0)
 		{
-			PartitionDesc partdesc = RelationGetPartitionDesc(rel);
 			int			nparts = partdesc->nparts;
 			Oid		   *part_oids = palloc(sizeof(Oid) * nparts);
 			bool		invalidate_parent = false;