JKISS32 bugfix: _y should never be zero

This is for the same reason as the similar protection
already in place for JLKISS64: _y is an independent XorShift
sub-generator, so it will be stuck at zero forever if it's
initialized to zero.
Merge pull request #1 from blblack/jkiss32fix

JKISS32 bugfix: _y should never be zero