Ensure that the Walker.pParse structure is initialized when walking the

source tree to gather the WINDOW clauses on a query with window functions.

FossilOrigin-Name: 4ca9d5d53d41d08fbce29f9da8cc0948df9c4c3136210af88b499cf889b5ccb8

src/expr.c

@@ -1363,6 +1363,7 @@ static void gatherSelectWindows(Select *p){
   w.xExprCallback = gatherSelectWindowsCallback;
   w.xSelectCallback = gatherSelectWindowsSelectCallback;
   w.xSelectCallback2 = 0;
+  w.pParse = 0;
   w.u.pSelect = p;
   sqlite3WalkSelect(&w, p);
 }