[PATCH] Improve AtSubCommit_childXids

Hi,
Surely that "s->nChildXids > 0", protects s->childXids to be NULL!
But, when we exchange the test (s->nChildXids > 0) by (s->childXids != NULL), I believe we have the same protection,
because,if "s->childXids" is not NULL, "s->nChildXids" is > 0, naturally. 

That way we can improve the function and avoid calling and setting unnecessarily!

Bonus: silent compiler warning potential null pointer derenferencing.

Best regards,
Ranier Vilela

--- \dll\postgresql-12.0\a\backend\access\transam\xact.c    Mon Sep 30 17:06:55 2019
+++ xact.c    Wed Nov 13 13:03:28 2019
@@ -1580,20 +1580,20 @@
      */
     s->parent->childXids[s->parent->nChildXids] = XidFromFullTransactionId(s->fullTransactionId);

-    if (s->nChildXids > 0)
+    if (s->childXids != NULL) {
         memcpy(&s->parent->childXids[s->parent->nChildXids + 1],
                s->childXids,
                s->nChildXids * sizeof(TransactionId));
+        /* Release child's array to avoid leakage */
+        pfree(s->childXids);

+        /* We must reset these to avoid double-free if fail later in commit */
+        s->childXids = NULL;
+        s->nChildXids = 0;
+        s->maxChildXids = 0;
+    }
+    Assert(s->nChildXids == 0 && s->maxChildXids == 0);
     s->parent->nChildXids = new_nChildXids;
-
-    /* Release child's array to avoid leakage */
-    if (s->childXids != NULL)
-        pfree(s->childXids);
-    /* We must reset these to avoid double-free if fail later in commit */
-    s->childXids = NULL;
-    s->nChildXids = 0;
-    s->maxChildXids = 0;
 }

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