signal.c: check the next page too

* signal.c (check_stack_overflow): check the next page too.  SP in
  ucontext is not decremented yet when `push` failed, so the fault
  page can be the next.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45924 b2dd03c8-39d4-4d8f-98ff-823fe69b080e