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No the problem is probably here that if there are NO 2 centroids, then the norm function will indeed give you problems. Add the following check before these lines of code.

if(mc.size() < 2){
   cerr << "not enough points to match." << endl;
   return -1;
}

This line will check if their is enough data for the norm function.

SOLUTION grabbed from comments:

To avoid the error the @LorenaGdL is talking about, you could add

if(!contours.empty()){
   Moments mom = cv::moments(contours[0]);
}

And then you will be safe nevertheless.

No the problem is probably here that if there are NO 2 centroids, then the norm function will indeed give you problems. Add the following check before these lines of code.

if(mc.size() < 2){
   cerr << "not enough points to match." << endl;
   return -1;
}

This line will check if their is enough data for the norm function.