eval_intern.h: avoid undefined behavior of setjmp

* eval_intern.h (TH_EXEC_TAG, TH_JUMP_TAG): get rid of undefined
  behavior of setjmp() in rhs of assignment expression.
  [ISO/IEC 9899:1999] 7.13.1.1

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43522 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-11-04

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43523 b2dd03c8-39d4-4d8f-98ff-823fe69b080e