Ensure regionalEndpoint variable is declared for s3UsEast1RegionalEnd…

…point (#4531)

If the file is executed in node strict mode https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode#assigning_to_undeclared_variables the line fails. Typically that should not happen because the file does not declare a "use strict" at the top but with some bundlers it can still happen.

lib/services/s3.js

@@ -278,7 +278,7 @@ AWS.util.update(AWS.S3.prototype, {
       req.httpRequest.endpoint.hostname.indexOf('s3.amazonaws.com') >= 0
     ) {
       var insertPoint = config.endpoint.indexOf('.amazonaws.com');
-      regionalEndpoint = config.endpoint.substring(0, insertPoint) +
+      var regionalEndpoint = config.endpoint.substring(0, insertPoint) +
         '.us-east-1' + config.endpoint.substring(insertPoint);
       req.httpRequest.updateEndpoint(regionalEndpoint);
     }