Fix bug: use return instead of next

return goes back one frame up the stack
next goes back to the closest frame on the stack that supports the
operation (that is close enough in RefSeqGPFFParser alone)
It works unless I subclass create_xrefs, and then my Hive workers die:

Lost control. Check your Runnable for loose 'next' statements that are
not part of a loop       WORKER_ERROR