ppp_net: Make static analysis tools happy

ppp_net: Make static analysis tools happy

The kernel simply puts a null terminator at index 15 prior to ifr_name
processing.  So we do the same.

Original report by:
Sabas Rosales, Blanca E <blanca.e.sabas.rosales@intel.com>

 Buffer not null terminated (BUFFER_SIZE_WARNING) buffer_size_warning:
 Calling strncpy with a maximum size argument of 16 bytes on destination
 array ifr.ifr_ifrn.ifrn_name of size 16 bytes might leave the
 destination string unterminated.

  67        strncpy(ifr.ifr_name, net->if_name, sizeof(ifr.ifr_name));