vsnprintf.c: initialize cp

* vsnprintf.c (BSD_vfprintf): initialize cp so that size is 0 in the
  commented case.  fix an accidental bug at r16716.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42977 b2dd03c8-39d4-4d8f-98ff-823fe69b080e