Fix PHP 7.1 Warning: illegal string offset 'src' (#6118)

* Simple fix for warning about illegal string offset

* Simplify how image array is built from post/author data

* Fix typo in 51e8a8f

* Replace direct use of `$post` with Core helpers

Cleans up the function to remove a global declared inside a conditional, where it could be overlooked. Also, helper functions FTW!

functions.opengraph.php

@@ -261,22 +261,18 @@ function jetpack_og_tags() {
 }
 
 function jetpack_og_get_image( $width = 200, $height = 200, $max_images = 4 ) { // Facebook requires thumbnails to be a minimum of 200x200
-	$image = '';
+	$image = array();
 
 	if ( is_singular() && ! is_home() ) {
-		global $post;
-		$image = '';
-
-		// Grab obvious image if $post is an attachment page for an image
-		if ( is_attachment( $post->ID ) && 'image' == substr( $post->post_mime_type, 0, 5 ) ) {
-			$image = wp_get_attachment_url( $post->ID );
+		// Grab obvious image if post is an attachment page for an image
+		if ( is_attachment( get_the_ID() ) && 'image' == substr( get_post_mime_type(), 0, 5 ) ) {
+			$image['src'] = wp_get_attachment_url( get_the_ID() );
 		}
 
 		// Attempt to find something good for this post using our generalized PostImages code
-		if ( ! $image && class_exists( 'Jetpack_PostImages' ) ) {
-			$post_images = Jetpack_PostImages::get_images( $post->ID, array( 'width' => $width, 'height' => $height ) );
+		if ( empty( $image ) && class_exists( 'Jetpack_PostImages' ) ) {
+			$post_images = Jetpack_PostImages::get_images( get_the_ID(), array( 'width' => $width, 'height' => $height ) );
 			if ( $post_images && ! is_wp_error( $post_images ) ) {
-				$image = array();
 				foreach ( (array) $post_images as $post_image ) {
 					$image['src'] = $post_image['src'];
 					if ( isset( $post_image['src_width'], $post_image['src_height'] ) ) {
@@ -309,14 +305,6 @@ function jetpack_og_get_image( $width = 200, $height = 200, $max_images = 4 ) {
 		}
 	}
 
-	if ( empty( $image ) ) {
-		$image = array();
-	} else if ( ! is_array( $image ) ) {
-		$image = array(
-			'src' => $image
-		);
-	}
-
 	// First fall back, blavatar
 	if ( empty( $image ) && function_exists( 'blavatar_domain' ) ) {
 		$blavatar_domain = blavatar_domain( site_url() );