patch 8.2.4924: maparg() may return a string that cannot be reused

Problem:    maparg() may return a string that cannot be reused.
Solution:   use msg_outtrans_special() instead of str2special().
            (closes #10384)

src/testdir/test_mapping.vim

@@ -502,6 +502,13 @@ func Test_list_mappings()
   call assert_equal(['n  <M-…>         foo'],
         \ execute('nmap <M-…>')->trim()->split("\n"))
 
+  " illegal bytes
+  let str = ":\x7f:\x80:\x90:\xd0:"
+  exe 'nmap foo ' .. str
+  call assert_equal(['n  foo           ' .. strtrans(str)],
+        \ execute('nmap foo')->trim()->split("\n"))
+  unlet str
+
   " map to CTRL-V
   exe "nmap ,k \<C-V>"
   call assert_equal(['n  ,k            <Nop>'],