signal.c: clear received signal at stack overflow

* signal.c (check_stack_overflow): clear the received reserved
  signal before raising stack overflow but not aborting.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48910 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread.c: no allocation during GC

* thread.c (ruby_thread_stack_overflow): jump without setting up
  the exception not to allocate new objects, during GC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48911 b2dd03c8-39d4-4d8f-98ff-823fe69b080e