extra modules check break demo

[zivyuan]

https://github.com/justadudewhohacks/opencv4nodejs/blob/24bff921c8d9c78500228e4be2d7bcd36e7a3904/examples/matchFeatures.js#L34

cv.modules was undefined on my Mac, add a undefined check to fix this problem

if (cv.xmodules && cv.xmodules.xfeatures2d) {

[UrielCh]

I did not xmodules define anywhere in the code, I had to define a new one in my fork the change

the fork: @u4/opencv4nodejs