node-triton#94 Protect against cache lookup returning null

If the cache file does not exist _cacheGetJson will not pass any value to the callback.

In the getImage function the callback passed to _cacheGetJson assumes if there was no error value then the second argument must be an array.

Add some logic to guard against null values before iterating over the array.

lib/tritonapi.js

@@ -354,12 +354,14 @@ TritonApi.prototype.getImage = function getImage(opts, cb) {
                         next(err);
                         return;
                     }
+                    if (images) {
                         for (var i = 0; i < images.length; i++) {
                             if (images[i].id === opts.name) {
                                 img = images[i];
                                 break;
                             }
                         }
+                    }
                     next();
                 });
             },