Null Regexp.warnings field so it doesn't contain hard reference to the runtime.
Null Regexp.warnings field so it doesn't contain hard reference to the runtime.

git-svn-id: 0c916ba55c552368470901e109eaaafe5410401d@6090 961051c9-f516-0410-bf72-c9f7e237a7b7

jruby/joni