Check for image before calling imagecreatefromstring()

The current code can run into a few conditions where the end result is that `imagecreatefromstring()` is called on something that isn't actually an image.

I found two conditions where that happens.  The first is where we end up with a NULL value at the end.  The second is where a remote image is requested, but what you get back is something else ( like the HTML for an error page ).

This update adds checks to defend against both of those possible error conditions.

projects/plugins/jetpack/_inc/lib/tonesque.php

@@ -86,7 +86,10 @@ public static function imagecreatefromurl( $image_url ) {
 
 			if ( empty( $data ) ) {
 				$response = wp_safe_remote_get( $image_url );
-				if ( is_wp_error( $response ) ) {
+				if ( 
+					is_wp_error( $response )
+					|| ! wp_startswith( $response['headers']['content-type'], 'image/' )
+				) {
 					return false;
 				}
 				$data = wp_remote_retrieve_body( $response );
@@ -103,6 +106,10 @@ public static function imagecreatefromurl( $image_url ) {
 			}
 		}
 
+		if ( null === $data ) {
+			return false;
+		}
+
 		// Now turn it into an image and return it.
 		return imagecreatefromstring( $data );
 	}