publishcontent_tab_load() does not check if node exists. [#2782937]

When visiting a publishcontent path for a non-existing node, the following message appears:

Notice: Trying to get property of non-object in publishcontent_tab_load() (line 70 of ../sites/all/modules/contrib/publishcontent/publishcontent.module).

Usually one will not visit these pages anyway, but it still there should be a robust behavior in case someone does.
I personally had this happen with devel "show redirection page", After deleting a node. But one can artificially reproduce this by visiting an url like 'node/123/publish/xyz', for a node id that does not exist.

Code now:

function publishcontent_tab_load($nid) {
  if (is_numeric($nid)) {
    $node = node_load($nid);
    if (variable_get('publishcontent_' . $node->type, FALSE)) {
      return $node;
    }
  }
  return FALSE;
}

should be replaced with:

function publishcontent_tab_load($nid) {
  if ($node = node_load($nid)) {
    if (variable_get('publishcontent_' . $node->type, FALSE)) {
      return $node;
    }
  }
  return FALSE;
}

I think the is_numeric() is not really necessary. Core's node_load() is already used as a wildcard loader, so it should be safe enough.