Fix mark stack overflow checking in push_selected

Fix mark stack overflow checking in push_selected

* mark.c (GC_push_selected): In case of a danger of mark stack overflow
after the first GC_push_all() call then call GC_push_all(bottom, top)
and return; remove redundant checking of GC_mark_stack_top at the end
of the function (overflow is already checked in GC_push_all).