Media: Sanity check image meta in 'wp_image_src_get_dimensions'.

This fixes a potential illegal offset error introduced in [50134] if the `$image_meta` doesn't include a `file` key.
    
Props dd32.
Fixes #51865.

Built from https://develop.svn.wordpress.org/trunk@50136


git-svn-id: http://core.svn.wordpress.org/trunk@49815 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media.php

@@ -1602,7 +1602,10 @@ function wp_image_src_get_dimensions( $image_src, $image_meta, $attachment_id =
 	$dimensions = false;
 
 	// Is it a full size image?
-	if ( strpos( $image_src, $image_meta['file'] ) !== false ) {
+	if (
+		isset( $image_meta['file'] ) &&
+		strpos( $image_src, $image_meta['file'] ) !== false
+	) {
 		$dimensions = array(
 			(int) $image_meta['width'],
 			(int) $image_meta['height'],

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-50135';
+$wp_version = '5.7-alpha-50136';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.