Fix filebase corruption caused by FileBase.renew()

echicken reported this problem: pretty severe filebase corruption as a result
of calling FileBase.renew(). The problem seemed pretty obvious (only loading
the file's index record), but I do think the underlying SMBLIB functions
should be more resilient to misuse and not corrupt the base. So, that's still
a TODO.

No existing scripts were using FileBase.renew(), so this problem has gone
unnoticed since SBBS v3.19.