When creating a HTAB we need to use HASH_COMPARE flag in order to set a user defined comparison function. (#6845)

DESCRIPTION: Fixes memory errors, caught by valgrind, of type
"conditional jump or move depends on uninitialized value"

When running Citus tests under Postgres with valgrind, the test cases
calling into `NonBlockingShardSplit` function produce valgrind errors of
type "conditional jump or move depends on uninitialized value".

The issue is caused by creating a HTAB in a wrong way. HASH_COMPARE flag
should have been used when creating a HTAB with user defined comparison
function. In the absence of HASH_COMPARE flag, HTAB falls back into
built-in string comparison function. However, valgrind somehow discovers
that the match function is not assigned to the user defined function as
intended.

Fixes #6835

(cherry picked from commit e7a25d82c9)

src/backend/distributed/operations/shard_split.c

@@ -1810,7 +1810,7 @@ CreateWorkerForPlacementSet(List *workersForPlacementList)
 	/* we don't have value field as it's a set */
 	info.entrysize = info.keysize;
 
-	uint32 hashFlags = (HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);
+	uint32 hashFlags = (HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT | HASH_COMPARE);
 
 	HTAB *workerForPlacementSet = hash_create("worker placement set", 32, &info,
 											  hashFlags);