Fix for issue #2541

A simple fix for issue https://github.com/matplotlib/matplotlib/issues/2541

Explanation:
Say the plot has the 0 facing North and it goes counter clockwise. The offset is 0.5*pi (as set in the set_theta_zero_location() function). 
Then if the mouse is in the first quadrant, the theta variable is in the range of \[0, 0.5*pi] (before applying offset and direction). In line 152, we account for the offset by subtracting the offset amount.
```
theta -= theta_offset
```
This gives us a negative number, which is the problem.