Fix crash in AWSXMLRequestSerializer

On certain requests (retries for S3 uploads) in `constructURIandHeadersAndBody:rules:uriSchema:error` if `checkResourceIsReachableAndReturnError:` returns an error it will crash

This is a fix for https://github.com/aws/aws-sdk-ios/issues/95 and can be verified with this [sample project](https://github.com/danielrhammond/AWSSDK-Crash-Sample-Project)