vim-patch:9.1.0126: Internal error when using upper-case mark in getregion()

Problem:  Internal error when passing mark in another buffer to
          getregion().
Solution: Don't allow marks in another buffer (zeertzjq)

closes: vim/vim#14076

Internal error when passing mark in another buffer to getregion()

421b597470

src/nvim/eval.lua

@@ -4387,6 +4387,8 @@ M.funcs = {
         |visual-mode|, an empty list is returned.
       - If {pos1}, {pos2} or {type} is an invalid string, an empty
         list is returned.
+      - If {pos1} or {pos2} is a mark in different buffer, an empty
+        list is returned.
 
       Examples: >
       	:xnoremap <CR>