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.
diff --git a/src/testdir/test_visual.vim b/src/testdir/test_visual.vim
index e08883d..3a7d273 100644
--- a/src/testdir/test_visual.vim
+++ b/src/testdir/test_visual.vim
@@ -157,8 +157,12 @@
 
 " 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 @@
 	      \ '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.
diff --git a/src/version.c b/src/version.c
index a6f9abd..5850cba 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1579,
+/**/
     1578,
 /**/
     1577,