Sometimes a TK_COLUMN Expr node can have Expr.pTab==0 if it is a reference

to an expression column in an index on and expression.  Fix for ticket
[aa98619ad08ddcab].

FossilOrigin-Name: d0da791ba0edfb65186459345e43500d8364a086a5a1651d828fecc1a1dd1edb

src/expr.c

@@ -1973,8 +1973,8 @@ int sqlite3ExprCanBeNull(const Expr *p){
     case TK_BLOB:
       return 0;
     case TK_COLUMN:
-      assert( p->pTab!=0 );
       return ExprHasProperty(p, EP_CanBeNull) ||
+             p->pTab==0 ||  /* Reference to column of index on expression */
              (p->iColumn>=0 && p->pTab->aCol[p->iColumn].notNull==0);
     default:
       return 1;