ex_cmds.c: Fix bug in ex_z (#6557)

vim-patch:8.0.0571

test/functional/ex_cmds/print_commands_spec.lua

@@ -0,0 +1,12 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, eq, command, funcs =
+  helpers.clear, helpers.eq, helpers.command, helpers.funcs
+
+describe(':z^', function()
+  before_each(clear)
+
+  it('correctly sets the cursor after :z^', function()
+    command('z^')
+    eq(1, funcs.line('.'))
+  end)
+end)