ofxFaceTrackerThreaded Bug fixes

- Fixed crash on startup for ofxFaceTrackerThreaded for iOS/Slow devices by returning default datatypes instead of clashing against NULL's or empty vectors. 
Problem was the ofxFaceTrackerThreaded::setup was not completing before the rest of the app started using it. So if the oF app running in the other thread asks the ofxFaceTracker for information via a function which may lead to crash if variables were NULL or 0 in size. 

- Fixed uninitialised variable ‘failedMiddle’. 
Problem: Causing a false positive reading for the tracker.getFound() in the first loop in LLVM / iOS.
Solution: Making sure it is a true (so YES to fail) at the start fixes this.