fixes #456: use .Random.seed only if it exists in the global environment

e.g. the expr may not trigger the creation of .Random.seed, such as set.seed(NULL) under R 2.15.x, which will fail
introduce an equivalent function to achieve set.seed(NULL) in R 2.15.x
Merge pull request #499 from yihui/bugfix/456

Bugfix/456