process.c (ruby_fork_ruby): fix race in signal handling

	We must block signals before stopping timer-thread, otherwise
	signal handing may be delayed until (and if) another signal
	is received after timer-thread is restarted.

	[ruby-core:87622] [Bug #14868] [Bug #13916]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@64439 b2dd03c8-39d4-4d8f-98ff-823fe69b080e