Fix CGImageRef leak in ZXImage.
Fix CGImageRef leak in ZXImage.

Fixes #96

Kalvar/ZXingObjC,peacemoon/ZXingObjC,ufs-slizhik/ZXingObjC,TheLevelUp/ZXingObjC,Kalvar/ZXingObjC,rkpant/ZXingObjC,JeremyPacker/ZXingObjC,RockyAo/ZXingObjC,adan016/ZXingObjC,trsathya/ZXingObjC,benjohnde/ZXingObjC,elegion/ios-pods-ZXingObjC,jianwoo/ZXingObjC,cortices/ZXingObjC,Stocard/ZXingObjC,trsathya/ZXingObjC,zilaiyedaren/ZXingObjC,wuxsoft/ZXingObjC,xuxubin/ZXingObjC,jianwoo/ZXingObjC,jebai0521/ZXingObjC,TheLevelUp/ZXingObjC,RockyAo/ZXingObjC,benjohnde/ZXingObjC,wuxsoft/ZXingObjC,Stocard/ZXingObjC,JeremyPacker/ZXingObjC,adan016/ZXingObjC,cortices/ZXingObjC,fuzhiqiang1989/ZXingObjC,fanwenkai/ZXingObjC,elegion/ios-pods-ZXingObjC,zilaiyedaren/ZXingObjC,sdhzwm/ZXingObjC,fuzhiqiang1989/ZXingObjC,rkpant/ZXingObjC,ufs-slizhik/ZXingObjC,davidjackman/ZXingObjC,tejesh-alimilli/ZXingObjC,tejesh-alimilli/ZXingObjC,Kevin775263419/ZXingObjC,fanwenkai/ZXingObjC,peacemoon/ZXingObjC,davidjackman/ZXingObjC,jebai0521/ZXingObjC,xuxubin/ZXingObjC,Kevin775263419/ZXingObjC,YotrolZ/ZXingObjC,zeffon/ZXingObjC,rowliny/ZXingObjC,YotrolZ/ZXingObjC,rowliny/ZXingObjC,sdhzwm/ZXingObjC,zeffon/ZXingObjC