patch 8.0.1579: virtual replace test fails in GUI

Problem:    Virtual replace test fails in GUI.
Solution:   Don't save key options if they were not set.

src/testdir/test_visual.vim

@@ -157,8 +157,12 @@ endfunc
 
 " Test Virtual replace mode.
 func Test_virtual_replace()
-  let save_t_kD = &t_kD
-  let save_t_kb = &t_kb
+  if exists('&t_kD')
+    let save_t_kD = &t_kD
+  endif
+  if exists('&t_kb')
+    let save_t_kb = &t_kb
+  endif
   exe "set t_kD=\<C-V>x7f t_kb=\<C-V>x08"
   enew!
   exe "normal a\nabcdefghi\njk\tlmn\n    opq	rst\n\<C-D>uvwxyz"
@@ -189,8 +193,12 @@ func Test_virtual_replace()
 	      \ 'AB	IJKLMNO	QRst'], getline(12, 13))
   enew!
   set noai bs&vim
-  let &t_kD = save_t_kD
-  let &t_kb = save_t_kb
+  if exists('save_t_kD')
+    let &t_kD = save_t_kD
+  endif
+  if exists('save_t_kb')
+    let &t_kb = save_t_kb
+  endif
 endfunc
 
 " Test Virtual replace mode.

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1579,
 /**/
     1578,
 /**/