Avoid useless palloc during transaction commit

We can allocate the initial relations-to-drop array when first needed, instead of at function entry; this avoids allocating it when the function is not going to do anything, which is most of the time. Backpatch to 9.3, where this behavior was introduced by commit 279628a. There's more that could be done here, such as possible reworking of the code to avoid having to palloc anything, but that doesn't sound as backpatchable as this relatively minor change. Per complaint from Noah Misch in 20131031145234.GA621493@tornado.leadboat.com

Author: alvherre

src/backend/catalog/storage.c

@@ -314,8 +314,8 @@ smgrDoPendingDeletes(bool isCommit)
 PendingRelDelete *next;
 intnrels = 0,
 i = 0,
-maxrels = 8;
-SMgrRelation *srels = palloc(maxrels * sizeof(SMgrRelation));
+maxrels = 0;
+SMgrRelation *srels = NULL;
 
 prev = NULL;
 for (pending = pendingDeletes; pending != NULL; pending = next)
@@ -340,8 +340,13 @@ smgrDoPendingDeletes(bool isCommit)
 
 srel = smgropen(pending->relnode, pending->backend);
 
-/* extend the array if needed (double the size) */
-if (maxrels <= nrels)
+/* allocate the initial array, or extend it, if needed */
+if (maxrels == 0)
+{
+maxrels = 8;
+srels = palloc(sizeof(SMgrRelation) * maxrels );
+}
+else if (maxrels <= nrels)
 {
 maxrels *= 2;
 srels = repalloc(srels, sizeof(SMgrRelation) * maxrels);
@@ -361,10 +366,9 @@ smgrDoPendingDeletes(bool isCommit)
 
 for (i = 0; i < nrels; i++)
 smgrclose(srels[i]);
-}
-
-pfree(srels);
 
+pfree(srels);
+}
 }
 
 /*