thread.c: deadlock in backtrace

	* thread.c (unblock_function_set): check interrupts just once
	  during raising exceptions, as they are deferred since r16651.
	  [ruby-core:85939] [Bug #14577]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@62917 b2dd03c8-39d4-4d8f-98ff-823fe69b080e