i3bar: Do not kill watcher on EOF

Long description:

Wait for the child process to exit on its own before freeing
watcher-related resources.

i3bar shows the last received status line until the process exits.

Fixes a race condition where the termination signal was sometimes not
received to display a meaningful error message.

To apply this patch, use:
curl http://cr.i3wm.org/patch/423/raw.patch | git am

@@ -266,6 +266,8 @@ static int stdin_end_array(void *context) {

 /*

  * Helper function to read stdin

  *

+ * Returns NULL on EOF.

+ *

  */

 static unsigned char *get_buffer(ev_io *watcher, int *ret_buffer_len) {

     int fd = watcher->fd;

@@ -285,9 +287,7 @@ static unsigned char *get_buffer(ev_io *watcher, int *ret_buffer_len) {

             exit(EXIT_FAILURE);

         }

         if (n == 0) {

-            /* end of file, kill the watcher */

             ELOG("stdin: received EOF\n");

-            cleanup();

             *ret_buffer_len = -1;

             return NULL;

         }