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 #6130)

src/eval.c

@@ -3515,10 +3515,11 @@ get_string_tv(char_u **arg, typval_T *rettv, int evaluate)
 	{
 	    ++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
+	    // to 21 characters (3 * 6 for the char and 3 for a modifier):
-	    // space for 5 extra.
+	    // 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/testdir/test_functions.vim

@@ -2280,6 +2280,7 @@ 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
 
 " Test for screenattr(), screenchar() and screenchars() functions

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    824,
 /**/
     823,
 /**/