MKNetworkEngine -imageAtURL: completionHandler: errorHandler: does not check that its block parameters are defined before invoking them

[jdjennin]

This code generates an EXC_BAD_ACCESS crash due to the lack of surrounding if-statements of the form

if (imageFetchedBlock) imageFetchedBlock([completedOperation responseImage], ...

and

if (errorBlock) errorBlock(completedOperation, error);