patch 9.1.0001: when closing window, wincmd p may fail

Avoid `prevwin == curwin` when closing `curwin`

Problem:  When closing the current window (or when moving it to a tabpage), the
          previous window may refer to the new current window
          (`winnr() == winnr('#')`) if that window is selected as the
          new current window.

Solution: Set `prevwin = NULL` when switching away from an invalid `curwin` and
          the target window was the `prevwin`.
          (Sean Dewar)

related: #4537
closes: #13762

Signed-off-by: Sean Dewar <seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1,
 /**/
     0
 };