patch 8.0.0342: double free with EXITFREE and setting 'ttytype'

Problem:    Double free when compiled with EXITFREE and setting 'ttytype'.
Solution:   Avoid setting P_ALLOCED on 'ttytype'. (Dominique Pelle,
            closes #1461)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 31a87af..9ac46f2 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -235,3 +235,22 @@
   call assert_fails("set showbreak=\x01", 'E595:')
   call assert_fails('set t_foo=', 'E846:')
 endfunc
+
+func Test_set_ttytype()
+  if !has('gui_running') && has('unix')
+    " Setting 'ttytype' used to cause a double-free when exiting vim and
+    " when vim is compiled with -DEXITFREE.
+    set ttytype=ansi
+    call assert_equal('ansi', &ttytype)
+    call assert_equal(&ttytype, &term)
+    set ttytype=xterm
+    call assert_equal('xterm', &ttytype)
+    call assert_equal(&ttytype, &term)
+    " FIXME: "set ttytype=" gives E522 instead of E529
+    " in travis on some builds. Why? Commented out this test for now.
+    " call assert_fails('set ttytype=', 'E529:')
+    call assert_fails('set ttytype=xxx', 'E522:')
+    set ttytype&
+    call assert_equal(&ttytype, &term)
+  endif
+endfunc