Don't display a byline if the page has an invalid author

Posts having post_author ids that refer to deleted users cause
problems - even if co-authors is populated.

In the short term, don't attempt to resolve the author, and
independently find posts with

a) deleted authors
b) populated co-authors

and switch them to be user 0

lib/byline.php

@@ -4,11 +4,17 @@
 function gds_byline()
 {
 	?>
-	<span class="govuk-visually-hidden">Posted by: </span>
-	<?php
-	if (function_exists('coauthors_posts_links')) {
-		coauthors_posts_links(', ');
+	<span class="govuk-visually-hidden">Posted by: </span><?php
+	$tmp_author_id = get_post_field( 'post_author', $post->ID );
+	$tmp_user =  get_user_by('id', $author_id );
+	if (empty($tmp_user->user_login)) {
+		echo "Deleted user $tmp_author_id";
+		error_log("{$post->ID} has deleted user $tmp_author_id", 0);
 	} else {
-		?> <a href="<?php echo get_author_posts_url(get_the_author_meta('ID')) ?>" rel="author" class="govuk-link"><?php echo get_the_author() ?></a> <?php
+		if (function_exists('coauthors_posts_links')) {
+			coauthors_posts_links(', ');
+		} else {
+			?> <a href="<?php echo get_author_posts_url(get_the_author_meta('ID')) ?>" rel="author" class="govuk-link"><?php echo get_the_author() ?></a> <?php
+		}
 	}
 }