Added checks to avoid 404 error

Added a check to avoid a 404 error which happened if the user didn’t
automatically accept webcam access. The `src` attribute of the `video`
tag was set to `undefined`, which led to a 404 error on the URL
`http://domain.tld/path_with_scanner/undefined`. It is now just set to
an empty string.

dist/quagga.js

@@ -1409,7 +1409,7 @@ define('input_stream',["image_loader"], function(ImageLoader) {
 
         that.setInputStream = function(config) {
             _config = config;
-            video.src = config.src;
+            video.src = (typeof config.src !== 'undefined') ? config.src : '';
         };
 
         that.ended = function() {

src/input_stream.js

@@ -29,7 +29,7 @@ define(["image_loader"], function(ImageLoader) {
 
         that.setInputStream = function(config) {
             _config = config;
-            video.src = config.src;
+            video.src = (typeof config.src !== 'undefined') ? config.src : '';
         };
 
         that.ended = function() {