Improved detection of corrupt node IDs in RTree shadow tables.

Test case in TH3.

Author: D. Richard Hipp

ext/rtree/rtree.c

@@ -761,6 +761,9 @@ static int nodeAcquire(
       rc = SQLITE_CORRUPT_VTAB;
       RTREE_IS_CORRUPT(pRtree);
     }
+  }else if( iNode<=0 ){
+    RTREE_IS_CORRUPT(pRtree);
+    rc = SQLITE_CORRUPT_VTAB;
   }else if( pRtree->iNodeSize==sqlite3_blob_bytes(pRtree->pNodeBlob) ){
     pNode = (RtreeNode *)sqlite3_malloc64(sizeof(RtreeNode)+pRtree->iNodeSize);
     if( !pNode ){