Use consistent declaration of oc_state_loop_filter_frag_rows_c().

The header used signed char _bf[256], make sure the method uses the
same.  Adjust pointer arithmetic to use array notation to make operation
explicit.

Avoid the following compiler message:

../../lib/state.c:1056:15: warning: argument 2 of type 'signed char *' declared as a pointer [-Warray-parameter=]
 1056 |  signed char *_bv,int _refi,int _pli,int _fragy0,int _fragy_end){
      |  ~~~~~~~~~~~~~^~~
In file included from ../../lib/state.c:20:
../../lib/state.h:526:14: note: previously declared as an array 'signed char[256]'
  526 |  signed char _bv[256],int _refi,int _pli,int _fragy0,int _fragy_end);
      |  ~~~~~~~~~~~~^~~~~~~~