patch 8.2.4419: illegal memory access when using 20 highlights

Problem:    Illegal memory access when using exactly 20 highlights.
Solution:   Add one more item in the array. (Brandon Richardson,
            closes #9800)

src/buffer.c

@@ -4170,8 +4170,11 @@ build_stl_str_hl(
     {
 	stl_items = ALLOC_MULT(stl_item_T, stl_items_len);
 	stl_groupitem = ALLOC_MULT(int, stl_items_len);
-	stl_hltab  = ALLOC_MULT(stl_hlrec_T, stl_items_len);
-	stl_tabtab = ALLOC_MULT(stl_hlrec_T, stl_items_len);
+
+	// Allocate one more, because the last element is used to indicate the
+	// end of the list.
+	stl_hltab  = ALLOC_MULT(stl_hlrec_T, stl_items_len + 1);
+	stl_tabtab = ALLOC_MULT(stl_hlrec_T, stl_items_len + 1);
     }
 
 #ifdef FEAT_EVAL
@@ -4251,11 +4254,13 @@ build_stl_str_hl(
 	    if (new_groupitem == NULL)
 		break;
 	    stl_groupitem = new_groupitem;
-	    new_hlrec = vim_realloc(stl_hltab, sizeof(stl_hlrec_T) * new_len);
+	    new_hlrec = vim_realloc(stl_hltab,
+					  sizeof(stl_hlrec_T) * (new_len + 1));
 	    if (new_hlrec == NULL)
 		break;
 	    stl_hltab = new_hlrec;
-	    new_hlrec = vim_realloc(stl_tabtab, sizeof(stl_hlrec_T) * new_len);
+	    new_hlrec = vim_realloc(stl_tabtab,
+					  sizeof(stl_hlrec_T) * (new_len + 1));
 	    if (new_hlrec == NULL)
 		break;
 	    stl_tabtab = new_hlrec;

src/testdir/test_tabline.vim

@@ -134,6 +134,17 @@ func Test_tabline_empty_group()
   set tabline=
 endfunc
 
+" When there are exactly 20 tabline format items (the exact size of the
+" initial tabline items array), test that we don't write beyond the size
+" of the array.
+func Test_tabline_20_format_items_no_overrun()
+  set showtabline=2
 
+  let tabline = repeat('%#StatColorHi2#', 20)
+  let &tabline = tabline
+  redrawtabline
+
+  set showtabline& tabline&
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4419,
 /**/
     4418,
 /**/