Ensure the caption shortcode outputs a valid HTML ID.

fixes #33179.

Built from https://develop.svn.wordpress.org/trunk@33489


git-svn-id: http://core.svn.wordpress.org/trunk@33456 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media.php

@@ -896,7 +896,7 @@ function img_caption_shortcode( $attr, $content = null ) {
 		return $content;
 
 	if ( ! empty( $atts['id'] ) )
-		$atts['id'] = 'id="' . esc_attr( $atts['id'] ) . '" ';
+		$atts['id'] = 'id="' . esc_attr( sanitize_html_class( $atts['id'] ) ) . '" ';
 
 	$class = trim( 'wp-caption ' . $atts['align'] . ' ' . $atts['class'] );