test_backtrace.rb: r18554 tests

* test/ruby/test_backtrace.rb (test_core_backtrace_{alias,undef}):
  add tests for r18554.  [ruby-dev:35820] [Bug #416]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46036 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
vm.c: rewind cfp

* vm.c (m_core_hash_{from_ary,merge_{ary,ptr,kwd}}): rewind cfp to
  show proper backtrace.  [ruby-dev:35820] [Bug #416]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46037 b2dd03c8-39d4-4d8f-98ff-823fe69b080e