fix exception when no focal length found
fix exception when no focal length found

mapillary/OpenSfM,BrookRoberts/OpenSfM,sunbingfengPI/OpenSFM_Test,mapillary/OpenSfM,sunbingfengPI/OpenSFM_Test,oscarlorentzon/OpenSfM,BrookRoberts/OpenSfM,mapillary/OpenSfM,sunbingfengPI/OpenSFM_Test,BrookRoberts/OpenSfM,BrookRoberts/OpenSfM,BrookRoberts/OpenSfM,oscarlorentzon/OpenSfM,oscarlorentzon/OpenSfM,sunbingfengPI/OpenSFM_Test,oscarlorentzon/OpenSfM,sunbingfengPI/OpenSFM_Test,mapillary/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM