fix var type issue which breaks crf_decode

CRF decode can fail when default type of "0" (as viewed by math_ops.maximum) does not match the type of sequence_length. 

This change is parallel in motivation and solution to the fix in https://github.com/tensorflow/tensorflow/commit/c0f1080188c5c6955cfa3b3c086ac262b1e5ec02, for crf_log_norm()=>_multi_seq_fn().