Fixed crashes on resize with keepingCropAspectRatio and cropAspectRatio

# Problem
Caused crash when crop window moved left bottom or right bottom corners with below settings.

```objc
PECropViewController *controller = [[PECropViewController alloc] init];
controller.toolbarHidden = YES;
controller.cropAspectRatio = 1.0;
controller.keepingCropAspectRatio = YES;
```

This crash caused by NaN value insert into view frame.

![ios mar 6 2014 5 06 33 pm](https://f.cloud.github.com/assets/113420/2343026/4be5a2d2-a506-11e3-82fa-e9967b29094e.png)

# Solution
In keepingCropAspectRatio = YES condition, constrained minWidth result effected minHeight result. I solved this problem by remove this dependency.