Fix compilation errors with Xcode 8.
Fix compilation errors with Xcode 8.

In its new SDK, the following APIs have been modified.

- UIGraphicsGetImageFromCurrentImageContext returns nil if the current context is nil or was not created by a call to UIGraphicsBeginImageContext.
- CGImage property of UIImage returns nil if the image is created from CIImage.

In the both of the cases, nil is never returned in SNTrim unless an image created from CIImage is assigned at the beginning. To check this unsupported condition, precondition has been added to image property (didSet) of SNTrimController.

snakajima/SNTrim