Fix CVE-2021-32492: Out-of-Bounds Read
https://sourceforge.net/p/djvu/djvulibre-git/ci/cd8b5c97b27a5c1dc83046498b6ca49ad20aa9b6

Index: libdjvu/DataPool.cpp
--- libdjvu/DataPool.cpp.orig
+++ libdjvu/DataPool.cpp
@@ -791,6 +791,9 @@ DataPool::create(const GP<DataPool> & pool, int start,
   DEBUG_MSG("DataPool::DataPool: pool=" << (void *)((DataPool *)pool) << " start=" << start << " length= " << length << "\n");
   DEBUG_MAKE_INDENT(3);
 
+  if (!pool)
+    G_THROW( ERR_MSG("DataPool.zero_DataPool") );
+
   DataPool *xpool=new DataPool();
   GP<DataPool> retval=xpool;
   xpool->init();
