Media: Avoid a PHP warning in `wp_calculate_image_srcset()` if a plugin returns a non-array value via `wp_calculate_image_srcset` filter.

Props mrmadhat, codegrau.
Fixes #43201.
Built from https://develop.svn.wordpress.org/trunk@42639


git-svn-id: http://core.svn.wordpress.org/trunk@42468 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media.php

@@ -1205,7 +1205,7 @@ function wp_calculate_image_srcset( $size_array, $image_src, $image_meta, $attac
 	$sources = apply_filters( 'wp_calculate_image_srcset', $sources, $size_array, $image_src, $image_meta, $attachment_id );
 
 	// Only return a 'srcset' value if there is more than one source.
-	if ( ! $src_matched || count( $sources ) < 2 ) {
+	if ( ! $src_matched || ! is_array( $sources ) || count( $sources ) < 2 ) {
 		return false;
 	}
 

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-42638';
+$wp_version = '5.0-alpha-42639';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.