Fix a code-generator issue associated with very unusual use of window

functions.  Both the expr.c or the window.c changes will each
independently fix the problem.  They are both included in this patch for
defense in depth.  [forum:/forumpost/0d48347967|Forum post 0d48347967].

FossilOrigin-Name: 1ba22631a7831e3562eda0eb6a5edf7f009c85c7ab4451d9eacd13ef0fb6036a

src/window.c

@@ -785,6 +785,7 @@ static int selectWindowRewriteExprCb(Walker *pWalker, Expr *pExpr){
       }
       /* no break */ deliberate_fall_through
 
+    case TK_IF_NULL_ROW:
     case TK_AGG_FUNCTION:
     case TK_COLUMN: {
       int iCol = -1;