sys-file-reader: Fix double-free on read of partial record.

sys-file-reader: Fix double-free on read of partial record.

partial_record() does a longjmp() to code near the top of
sys_file_casereader_read() that unrefs the case, but this code at the
end of sys_file_casereader_read() would have already unref'ed it once in
that case, resulting in a double-free.