The problem is with this line:
or results_x(i,3)=mean(tstat_x_star(:,i)<results_x(i,2));
I cannot run your code to test this, but this code will at least not throw that error:
for i=1:N_x+k_x
results_x(i,3) = mean(tstat_x_star(:,i)>results_x(i,2)) | mean(tstat_x_star(:,i)<results_x(i,2));
disp(i);
end