vim-patch:8.2.2476: using freed memory when splitting window while closing buffer

Problem:    Using freed memory when using an autocommand to split a window
            while a buffer is being closed.
Solution:   Disallow splitting when the buffer has b_locked_split set.
983d83ff1c

Put the error message in window.c.
Cherry-pick a memory leak fix from Vim patch 8.2.0399.
Test still fails.

src/nvim/ex_getln.c

@@ -6376,6 +6376,7 @@ static int open_cmdwin(void)
   // Create a window for the command-line buffer.
   if (win_split((int)p_cwh, WSP_BOT) == FAIL) {
     beep_flush();
+    ga_clear(&winsizes);
     return K_IGNORE;
   }
   cmdwin_type = get_cmdline_type();