lib: Fix gcc 9 warning -> error issue

lib: Fix gcc 9 warning -> error issue

gcc is complaing about this with --enable-dev and --enable-werror:

In function 'nb_log_callback',
    inlined from 'nb_transaction_apply_finish' at lib/northbound.c:1106:4:
lib/northbound.c:777:2: error: '%s' directive argument is null [-Werror=format-overflow=]
  777 |  zlog_debug(
      |  ^~~~~~~~~~~
  778 |   "northbound callback: event [%s] op [%s] xpath [%s] value [%s]",
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  779 |   nb_event_name(event), nb_operation_name(operation), xpath,
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  780 |   value);
      |   ~~~~~~
  CC       lib/ringbuf.lo

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>