vim-patch:8.2.0824: still not enough memory allocated when converting string

Problem:    Still not enough memory allocated when converting string with
            special character.
Solution:   Reserve space for expanding K_SPECIAL. (closes vim/vim#6130)
1919371b2b

src/nvim/eval.c

@@ -4527,10 +4527,11 @@ static int get_string_tv(char_u **arg, typval_T *rettv, int evaluate)
     if (*p == '\\' && p[1] != NUL) {
       p++;
       // A "\<x>" form occupies at least 4 characters, and produces up
-      // to 9 characters (6 for the char and 3 for a modifier): reserve
-      // space for 5 extra.
+      // to 21 characters (3 * 6 for the char and 3 for a modifier):
+      // reserve space for 18 extra.
+      // Each byte in the char could be encoded as K_SPECIAL K_EXTRA x.
       if (*p == '<') {
-        extra += 5;
+        extra += 18;
       }
     }
   }

src/nvim/testdir/test_functions.vim

@@ -1374,6 +1374,7 @@ func Test_nr2char()
   call assert_equal('a', nr2char(97, 0))
 
   call assert_equal("\x80\xfc\b\xf4\x80\xfeX\x80\xfeX\x80\xfeX", eval('"\<M-' .. nr2char(0x100000) .. '>"'))
+  call assert_equal("\x80\xfc\b\xfd\x80\xfeX\x80\xfeX\x80\xfeX\x80\xfeX\x80\xfeX", eval('"\<M-' .. nr2char(0x40000000) .. '>"'))
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab