MAXALIGN the target address where we store flattened value.

The API (EOH_flatten_into) that flattens the expanded value representation
expects the target address to be maxaligned.  All it's usage adhere to that
principle except when serializing datums for parallel query.  Fix that
usage.

Diagnosed-by: Tom Lane
Author: Tom Lane and Amit Kapila
Backpatch-through: 9.6
Discussion: https://postgr.es/m/11629.1536550032@sss.pgh.pa.us

src/backend/utils/adt/datum.c

@@ -338,8 +338,19 @@ datumSerialize(Datum value, bool isnull, bool typByVal, int typLen,
 		}
 		else if (eoh)
 		{
-			EOH_flatten_into(eoh, (void *) *start_address, header);
+			char	   *tmp;
+
+			/*
+			 * EOH_flatten_into expects the target address to be maxaligned,
+			 * so we can't store directly to *start_address.
+			 */
+			tmp = (char *) palloc(header);
+			EOH_flatten_into(eoh, (void *) tmp, header);
+			memcpy(*start_address, tmp, header);
 			*start_address += header;
+
+			/* be tidy. */
+			pfree(tmp);
 		}
 		else
 		{