The short answer is

1. a += 0.01;
2. a = (a > 0) ? a : a + TWO_PI;
3. a = a % TWO_PI;

It can be shortened further to a single instruction

1. a = (((a += 0.1) > 0) ? a : a + TWO_PI)% TWO_PI;

This proves that it works

1. float cx, cy, rx, ry, a;
2. float len = 100;
3. 
   
4. public void setup(){
5.   size(250, 250);
6.   cx = width/2;
7.   cy = height/2;
8.   rx = ry = 100;
9.   a = 0;
10.   strokeWeight(2);
11. }
12. 
    
13. public void draw(){
14.   background(255);
15.   rx = len * cos(a);
16.   ry = len * sin(a);
17.   line(cx, cy, cx + rx, cy + ry);
18.   a = (((a += 0.01) > 0) ? a : a + TWO_PI)% TWO_PI;
19. }

This problem is associated with values returned by the atan2() method