MFC: r300397

1. POSIX prohibits printing errors to stderr here and require

returning NULL:

"Upon successful completion, initstate() and setstate() shall return a
pointer to the previous state array; otherwise, a null pointer shall
be returned.

Although some implementations of random() have written messages to
standard error, such implementations do not conform to POSIX.1-2008."

2. Move error detections earlier to prevent state modifying.