VM: refactor bignum.cpp so that GC_BIGNUM is never called on a NULL pointer
VM: refactor bignum.cpp so that GC_BIGNUM is never called on a NULL
pointer

The purpose of this fix is to ensure that no NULL checks are needed when
traversing the bignum_roots vector.

dch/factor,AlexIljin/factor,slavapestov/factor,dch/factor,bjourne/factor,bjourne/factor,factor/factor,bpollack/factor,dch/factor,nicolas-p/factor,slavapestov/factor,bjourne/factor,factor/factor,bpollack/factor,mrjbq7/factor,dch/factor,slavapestov/factor,bjourne/factor,mrjbq7/factor,tgunr/factor,slavapestov/factor,bjourne/factor,bpollack/factor,slavapestov/factor,nicolas-p/factor,tgunr/factor,nicolas-p/factor,AlexIljin/factor,sarvex/factor-lang,tgunr/factor,factor/factor,tgunr/factor,factor/factor,AlexIljin/factor,slavapestov/factor,sarvex/factor-lang,AlexIljin/factor,nicolas-p/factor,dch/factor,nicolas-p/factor,bjourne/factor,dch/factor,mrjbq7/factor,bpollack/factor,sarvex/factor-lang,sarvex/factor-lang,AlexIljin/factor,mrjbq7/factor,bpollack/factor,nicolas-p/factor,factor/factor,sarvex/factor-lang,bjourne/factor,factor/factor,sarvex/factor-lang,AlexIljin/factor,tgunr/factor,mrjbq7/factor,mrjbq7/factor,slavapestov/factor,sarvex/factor-lang,bpollack/factor,tgunr/factor,bpollack/factor,AlexIljin/factor,nicolas-p/factor