Put an ALWAYS() on an unreachable branch in the sqlite3ExprListDup() routine.

FossilOrigin-Name: 2caaaab5e07f6b76d66fbe4e93a4b956937427d8
diff --git a/src/expr.c b/src/expr.c
index ebc9ca6..072c420 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -1311,6 +1311,7 @@
     Expr *pOldExpr = pOldItem->pExpr;
     Expr *pNewExpr;
     pItem->pExpr = sqlite3ExprDup(db, pOldExpr, flags);
+    assert( pItem->pExpr==0 || db->mallocFailed==0 );
     if( pOldExpr 
      && pOldExpr->op==TK_SELECT_COLUMN
      && (pNewExpr = pItem->pExpr)!=0 
@@ -1319,7 +1320,7 @@
       if( pNewExpr->iColumn==0 ){
         assert( pOldExpr->pLeft==pOldExpr->pRight );
         pNewExpr->pLeft = pNewExpr->pRight;
-      }else if( pItem[-1].pExpr!=0 ){
+      }else if( ALWAYS(pItem[-1].pExpr!=0) ){
         pNewExpr->pLeft = pItem[-1].pExpr->pLeft;
       }
     }