Patch to avoid passing invalid arg in getTruncationIndex() crashes on iOS 10 beta.
Patch to avoid passing invalid arg in getTruncationIndex() crashes on iOS 10 beta.

pivotaltracker/DTCoreText,Cocoanetics/DTCoreText,Cocoanetics/DTCoreText,Akylas/DTCoreText,rgregg/DTCoreText,rgregg/DTCoreText,Akylas/DTCoreText,Cocoanetics/DTCoreText