time.c (time_timespec): fix tv_nsec overflow

test/ruby/test_float.rb (test_sleep_with_Float) causes tv_nsec
to hit 1000000000 exactly.  This bug is currently hidden from
our test by the platform-dependent native_cond_timeout
functions, but a future native_cond_timedwait implementation may
prefer relative timeouts to match ConditionVariable#wait
semantics more closely.
[Bug #10144]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47196 b2dd03c8-39d4-4d8f-98ff-823fe69b080e