Check for a NULL value in *error.

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.