signal.c: get rid of heap corruption

* signal.c (rb_sigaltstack_size): double default size to get rid
  of heap corruption by alternate stack overflow in SEGV handler.
  typically happened at fprintf() in control_frame_dump().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47984 b2dd03c8-39d4-4d8f-98ff-823fe69b080e