Change the way a comparison used to detect corrupt databases in fts3 is done

to avoid potential pointer overflow in 32-bit builds.

FossilOrigin-Name: 95a9a39ff784b960b62dd6298e722a620ba3f9c8b76839a49bbef35d7bc84a8e

ext/fts3/fts3_write.c

@@ -1409,7 +1409,7 @@ static int fts3SegReaderNext(
   ** b-tree node. And that the final byte of the doclist is 0x00. If either 
   ** of these statements is untrue, then the data structure is corrupt.
   */
-  if( (&pReader->aNode[pReader->nNode] - pReader->aDoclist)<pReader->nDoclist
+  if( pReader->nDoclist-(pReader->aDoclist-pReader->aNode) > pReader->nNode
    || (pReader->nPopulate==0 && pReader->aDoclist[pReader->nDoclist-1])
   ){
     return FTS_CORRUPT_VTAB;