* vm_exec.c (cfp): Fixes a SEGV issue in r44554.
  r11 can be broken by subroutine and sometimes causes SEGV at
  runtime.  Use r13 instead.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44590 b2dd03c8-39d4-4d8f-98ff-823fe69b080e