use x$lastSeedValue, not .Random.seed (which may not exist)

[MichaelChirico]

I'm seeing an error where cbind() wounds up run without .Random.seed set, which this solves.

Per ?.Random.seed, no seed is set initially in the session until the first time set.seed() is run:

 Initially, there is no seed; a new one is created from the current time and the process ID when one is required.

I don't know enough about the package to know if this fix is for sure correct, but I noticed a similar fix applied in the same file:

https://github.com/amices/mice/commit/1189fe10aee92077f05d68279da7974090e86d60