Fix error handling of XLogReaderAllocate in case of OOM

Similarly to previous fix 9b8d478, commit 2c03216 has switched
XLogReaderAllocate() to use a set of palloc calls instead of malloc,
causing any callers of this function to fail with an error instead of
receiving a NULL pointer in case of out-of-memory error. Fix this by
using palloc_extended with MCXT_ALLOC_NO_OOM that will safely return
NULL in case of an OOM.

Michael Paquier, slightly modified by me.

src/backend/replication/logical/logical.c

@@ -163,6 +163,11 @@ StartupDecodingContext(List *output_plugin_options,
 	ctx->slot = slot;
 
 	ctx->reader = XLogReaderAllocate(read_page, ctx);
+	if (!ctx->reader)
+		ereport(ERROR,
+				(errcode(ERRCODE_OUT_OF_MEMORY),
+				 errmsg("out of memory")));
+
 	ctx->reader->private_data = ctx;
 
 	ctx->reorder = ReorderBufferAllocate();