More paranoia when handling responses

lib/http-api.js

@@ -111,6 +111,11 @@ module.exports.MatrixHttpApi.prototype = {
                     case global.XMLHttpRequest.DONE:
                         clearTimeout(xhr.timeout_timer);
 
+                        if (!xhr.responseText) {
+                            cb(new Error('No response body.'));
+                            return;
+                        }
+
                         var resp = JSON.parse(xhr.responseText);
                         if (resp.content_uri === undefined) {
                             cb(new Error('Bad response'));