Fix crasher when trying to create an animated image from malformed data.
Fix crasher when trying to create an animated image from malformed data.

caoer/JTSImageViewController,jonesgithub/JTSImageViewController,jaredsinclair/JTSImageViewController,Kingside/JTSImageViewController,mattprowse/JTSImageViewController,IdeasOnCanvas/JTSImageViewController,bulusoy/JTSImageViewController,mash/JTSImageViewController,jogabo/JTSImageViewController,GeorgesAlkhouri/JTSImageViewController,tokorom/JTSImageViewController,lintaoSuper/JTSImageViewController,ainopara/JTSImageViewController,hoanganh6491/JTSImageViewController,ywzw2013/JTSImageViewController,Ethan-Z/JTSImageViewController,ello/JTSImageViewController,FanPortal/JTSImageViewController,iosphere/JTSImageViewController,atone/JTSImageViewController,clooth/JTSImageViewController,ElfSundae/JTSImageViewController