Fix 'undefined' bug in fallback URLs (in library version)
Fix 'undefined' bug in fallback URLs (in library version)

riuson/pannellum,psteph801/psteph801.github.io,riuson/pannellum,quentinyang/pannellum,mpetroff/pannellum,quentinyang/pannellum,quentinyang/pannellum,ywjrg/pannellum,Benvii/pannellum,psteph801/psteph801.github.io,mpetroff/pannellum,Benvii/pannellum,Benvii/pannellum,Benvii/pannellum,psteph801/psteph801.github.io,ywjrg/pannellum,mpetroff/pannellum,quentinyang/pannellum,psteph801/psteph801.github.io,riuson/pannellum,mpetroff/pannellum,riuson/pannellum,ywjrg/pannellum,ywjrg/pannellum