Fix failure to consider failure cases in GetComboCommandId().

Failure to initially palloc the comboCids array, or to realloc it bigger
when needed, left combocid's data structures in an inconsistent state that
would cause trouble if the top transaction continues to execute.  Noted
while examining a user complaint about the amount of memory used for this.
(There's not much we can do about that, but it does point up that repalloc
failure has a non-negligible chance of occurring here.)

In HEAD/9.5, also avoid possible invocation of memcpy() with a null pointer
in SerializeComboCIDState; cf commit 13bba022.

src/backend/utils/time/combocid.c

@@ -208,6 +208,13 @@ GetComboCommandId(CommandId cmin, CommandId cmax)
 	{
 		HASHCTL		hash_ctl;
 
+		/* Make array first; existence of hash table asserts array exists */
+		comboCids = (ComboCidKeyData *)
+			MemoryContextAlloc(TopTransactionContext,
+							   sizeof(ComboCidKeyData) * CCID_ARRAY_SIZE);
+		sizeComboCids = CCID_ARRAY_SIZE;
+		usedComboCids = 0;
+
 		memset(&hash_ctl, 0, sizeof(hash_ctl));
 		hash_ctl.keysize = sizeof(ComboCidKeyData);
 		hash_ctl.entrysize = sizeof(ComboCidEntryData);
@@ -218,12 +225,20 @@ GetComboCommandId(CommandId cmin, CommandId cmax)
 								CCID_HASH_SIZE,
 								&hash_ctl,
 								HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);
+	}
+
+	/*
+	 * Grow the array if there's not at least one free slot.  We must do this
+	 * before possibly entering a new hashtable entry, else failure to
+	 * repalloc would leave a corrupt hashtable entry behind.
+	 */
+	if (usedComboCids >= sizeComboCids)
+	{
+		int			newsize = sizeComboCids * 2;
 
 		comboCids = (ComboCidKeyData *)
-			MemoryContextAlloc(TopTransactionContext,
-							   sizeof(ComboCidKeyData) * CCID_ARRAY_SIZE);
-		sizeComboCids = CCID_ARRAY_SIZE;
-		usedComboCids = 0;
+			repalloc(comboCids, sizeof(ComboCidKeyData) * newsize);
+		sizeComboCids = newsize;
 	}
 
 	/* Lookup or create a hash entry with the desired cmin/cmax */
@@ -242,20 +257,7 @@ GetComboCommandId(CommandId cmin, CommandId cmax)
 		return entry->combocid;
 	}
 
-	/*
-	 * We have to create a new combo cid. Check that there's room for it in
-	 * the array, and grow it if there isn't.
-	 */
-	if (usedComboCids >= sizeComboCids)
-	{
-		/* We need to grow the array */
-		int			newsize = sizeComboCids * 2;
-
-		comboCids = (ComboCidKeyData *)
-			repalloc(comboCids, sizeof(ComboCidKeyData) * newsize);
-		sizeComboCids = newsize;
-	}
-
+	/* We have to create a new combo cid; we already made room in the array */
 	combocid = usedComboCids;
 
 	comboCids[combocid].cmin = cmin;