Theme Customizer: Strip slashes when sanitizing previewed values. see #19910.

git-svn-id: https://develop.svn.wordpress.org/trunk@20028 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/class-wp-customize-setting.php

@@ -161,6 +161,7 @@ class WP_Customize_Setting {
 	 * @return mixed Null if an input isn't valid, otherwise the sanitized value.
 	 */
 	public function sanitize( $value ) {
+		$value = stripslashes_deep( $value );
 		return apply_filters( "customize_sanitize_{$this->id}", $value );
 	}