This dates back to the original copying over from ptp-noposix:

  89a46c6d time-wrs/wrs-socket.c: implementation of net functions for wrs arch

(but the bug was not in ptp-noposix).

It is exposed by the newer compiler (well, library) we are using.
But a calloc of 0 bytes should return NULL, to ensure it's not used...
Documentation disagrees:

       If size is 0, then malloc() returns either NULL, or a unique
       pointer value that can later be successfully passed to free().
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>