[ruby/date] Call rb_gc_register_mark_object after object allocation

It's possible that both half_days_in_day and day_in_nanoseconds are Ruby
objects, which means that creating day_in_nanoseconds may trigger GC.
Since half_days_in_day is not registered as a mark object until after
day_in_nanoseconds is allocated, the GC may reclaim half_days_in_day.

We can see this crash:

    ruby(rb_print_backtrace+0xb) [0x63a373c0] vm_dump.c:1105
    ruby(rb_vm_bugreport) vm_dump.c:1450
    ruby(rb_assert_failure_detail+0xdb) [0x6371d3a2] error.c:1216
    ruby(RB_FL_TEST_RAW+0x0) [0x6371d3d5] error.c:1192
    ruby(rb_assert_failure) (null):0
    ruby(rb_gc_impl_writebarrier+0xb4) [0x636f01e4] gc/default/default.c:6103
    ruby(pin_array_list_append+0x72) [0x638f9787] include/ruby/internal/gc.h:788
    ruby(rb_vm_register_global_object) vm.c:4713
    ruby(rb_gc_register_mark_object+0x3a) [0x6374144a] gc.c:3449
    .ext/i686-linux-gnu/date_core.so(Init_date_core+0x204) [0xdbec86c4] ext/date/date_core.c:9511
    .ext/i686-linux-gnu/date_core.so(Init_date_core) (null):0
    ruby(dln_load_and_init+0x71) [0x6392c541] dln.c:521
    ruby(dln_load_feature+0xd2) [0x6392c7d2] dln.c:566
    ruby(load_ext+0xc3) [0x637931b3] load.c:1210
    ruby(rb_vm_pop_frame+0x0) [0x638f80cd] vm.c:3120
    ruby(rb_vm_call_cfunc_in_box) vm.c:3122
    ruby(rb_long2num_inline+0x0) [0x637956f8] load.c:1353
    ruby(require_internal) load.c:1354
    ruby(rb_require_string_internal+0x60) [0x63795fa1] load.c:1457
    ruby(rb_require_string) load.c:1443

https://github.com/ruby/date/commit/cbec5948e0

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