patch 8.2.4428: crash when switching tabpage while in the cmdline window

Problem:    Crash when switching tabpage while in the cmdline window.
Solution:   Disallow switching tabpage when in the cmdline window.
diff --git a/src/usercmd.c b/src/usercmd.c
index d4a7a9c..1ff4d0d 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -141,11 +141,7 @@
     /*
      * Look for buffer-local user commands first, then global ones.
      */
-    gap =
-#ifdef FEAT_CMDWIN
-	is_in_cmdwin() ? &prevwin->w_buffer->b_ucmds :
-#endif
-	&curbuf->b_ucmds;
+    gap = &prevwin_curwin()->w_buffer->b_ucmds;
     for (;;)
     {
 	for (j = 0; j < gap->ga_len; ++j)
@@ -358,11 +354,7 @@
 get_user_commands(expand_T *xp UNUSED, int idx)
 {
     // In cmdwin, the alternative buffer should be used.
-    buf_T *buf =
-#ifdef FEAT_CMDWIN
-	is_in_cmdwin() ? prevwin->w_buffer :
-#endif
-	curbuf;
+    buf_T *buf = prevwin_curwin()->w_buffer;
 
     if (idx < buf->b_ucmds.ga_len)
 	return USER_CMD_GA(&buf->b_ucmds, idx)->uc_name;
@@ -386,11 +378,7 @@
     if (cmdidx == CMD_USER_BUF)
     {
 	// In cmdwin, the alternative buffer should be used.
-	buf_T *buf =
-#ifdef FEAT_CMDWIN
-		    is_in_cmdwin() ? prevwin->w_buffer :
-#endif
-		    curbuf;
+	buf_T *buf = prevwin_curwin()->w_buffer;
 
 	if (idx < buf->b_ucmds.ga_len)
 	    return USER_CMD_GA(&buf->b_ucmds, idx)->uc_name;
@@ -478,11 +466,7 @@
     garray_T	*gap;
 
     // In cmdwin, the alternative buffer should be used.
-    gap =
-#ifdef FEAT_CMDWIN
-	    is_in_cmdwin() ? &prevwin->w_buffer->b_ucmds :
-#endif
-	    &curbuf->b_ucmds;
+    gap = &prevwin_curwin()->w_buffer->b_ucmds;
     for (;;)
     {
 	for (i = 0; i < gap->ga_len; ++i)