nqp::readlinefh should never return null.
nqp::readlinefh should never return null.

Resolves 5 of the NullPointerException aborts in the Rakudo spectests,
leading to 4 of the affected files now passing in full.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp