vim-patch:9.1.0195: memleak with ex_drop, NULL dereference

Problem:  memleak with ex_drop(), NULL dereference
          (zeertzjq)
Solution: revert back to ex_rewind(), use curbuf instead of buf

fixes: vim/vim#14246
closes: vim/vim#14251

85a769d466

Co-authored-by: Christian Brabandt <cb@256bit.org>

test/functional/ex_cmds/drop_spec.lua

@@ -72,7 +72,7 @@ describe(':drop', function()
        lazy dogs                         |
       {0:~                                  }|*6
       {1:Xdrop_modified.txt [+]             }|
-      "Xdrop_modified.txt" [New]         |
+      :drop Xdrop_modified.txt           |
     ]])
   end)
 end)