Make a buffer a "u_char" buffer, to avoid the warning some compilers

Make a buffer a "u_char" buffer, to avoid the warning some compilers
give when a "u_char *" is assigned a pointer to the first element of
that buffer.

Set the "ni" variable in the RIP "version 0" handler before using it.
(Note, though, that RFC 1058 says, in section 3.4:

      0   Datagrams whose version number is zero are to be ignored.
          These are from a previous version of the protocol, whose
          packet format was machine-specific.

so it's not clear that a version number of 0 should be printed in that
fashion.)