Fix range exceeds data length crash (#554)
Fix range exceeds data length crash (#554)

* Fix range exceeds data length crash

If the data returned on to the TextToSpeech synthesize function is
empty, a crash would occur because there was no length checking.

* Fix range exceeds data length crash

Incorporate review comments.

* Fix range exceeds data length crash

Incorporate review comments

tad-iizuka/swift-sdk,watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk,watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk,tad-iizuka/swift-sdk,tad-iizuka/swift-sdk