patch 8.0.0493: crash with cd command with very long argument

Problem:    Crash with cd command with very long argument.
Solution:   Check for running out of space. (Dominique pending, closes #1576)

src/testdir/test_alot.vim

@@ -3,6 +3,7 @@
 
 set belloff=all
 source test_assign.vim
+source test_cd.vim
 source test_changedtick.vim
 source test_cursor_func.vim
 source test_delete.vim

src/testdir/test_cd.vim

@@ -0,0 +1,13 @@
+" Test for :cd
+
+func Test_cd_large_path()
+  " This used to crash with a heap write overflow.
+  call assert_fails('cd ' . repeat('x', 5000), 'E472:')
+endfunc
+
+func Test_cd_up_and_down()
+  let path = getcwd()
+  cd ..
+  exe 'cd ' . path
+  call assert_equal(path, getcwd())
+endfunc