Add NULL pointer check on NSError and remove unused code.

These two comments are in reference to Issue #1652.

Not checking for a NULL value in `*error` was causing an EXC_BAD_ACCESS exception when the block in the following selector returns NO: 

`+ (instancetype)expectationWithSourceKeyPath:(NSString *)sourceKeyPath destinationKeyPath:(NSString *)destinationKeyPath evaluationBlock:(RKMappingTestExpectationEvaluationBlock)evaluationBlock`

The second commit was to remove the assignment to `*error` on line 239; its immediately overwritten on line 254 so it seems pointless.