Fixed crash on keep minimizing crop rect with keepingCropAspectRatio

# Problem
Crash on keep minimizing crop rect with below setting.

```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.

![http://gifzo.net/BRqsOpJFrr4.gif](http://gifzo.net/BRqsOpJFrr4.gif)

# Solution
Change aspect fit timing to after constrain minWidth and minHeight.