Destroy old pty even if it's no longer running (#249)

For unknown reasons, the pty will occasionally quit running. In these
cases, we still want to remove it so that a fresh one can be created. We
don't actually need this check because error messages from `zle` and
`zpty` are redirected to /dev/null.

One sure way to kill all currently running pty's is to run `exit` in a
subshell. Even without zsh-autosuggestions loaded, the following works:

    % zmodload zsh/zpty
    % zpty -b foo cat
    % zpty -b bar cat
    % zpty
    (31689) bar: cat
    (31666) foo: cat
    % $(exit)
    % zpty
    (finished) bar: cat
    (finished) foo: cat

spec/async_spec.rb

@@ -26,6 +26,16 @@ context 'with asynchronous suggestions enabled' do
       end
     end
   end
+
+  describe 'exiting a subshell' do
+    it 'should not cause error messages to be printed' do
+      session.run_command('$(exit)')
+
+      sleep 1
+
+      expect(session.content).to eq('$(exit)')
+    end
+  end
 end