Fix walIteratorInit() so that it always leaves the iterator as a NULL pointer

if an OOM occurs.  This fixes an assertion fault introduced by 
check-in [044b0b65e716bff].

FossilOrigin-Name: e5ce256aa1f7a8ae995b79c9da895827bee5d8d0724fc15413ff203dc9e2602d

src/wal.c

@@ -1667,6 +1667,7 @@ static int walIteratorInit(Wal *pWal, u32 nBackfill, WalIterator **pp){
 
   if( rc!=SQLITE_OK ){
     walIteratorFree(p);
+    p = 0;
   }
   *pp = p;
   return rc;