patch 8.2.2508: cannot change the character displayed in non existing lines

Problem:    Cannot change the character displayed in non existing lines.
Solution:   Add the "eob" item to 'fillchars'. (closes #7832, closes #3820)
diff --git a/src/screen.c b/src/screen.c
index cd727ed..b3944a7 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4765,6 +4765,7 @@
 	{&fill_vert,	"vert"},
 	{&fill_fold,	"fold"},
 	{&fill_diff,	"diff"},
+	{&fill_eob,	"eob"},
     };
     static struct charstab lcstab[] =
     {
@@ -4812,7 +4813,10 @@
 		lcs_tab3 = NUL;
 	    }
 	    else
+	    {
 		fill_diff = '-';
+		fill_eob = '~';
+	    }
 	}
 	p = *varp;
 	while (*p)