liboping: fix format-truncation error

liboping: fix format-truncation error

liboping.c: In function 'ping_host_add':
liboping.c:207:9: error: '%s' directive output may be truncated writing
up to 255 bytes into a region of size 243 [-Werror=format-truncation=]
    "%s: %s", function, message);
         ^~
liboping.c:1644:40:
    ping_set_error (obj, "getaddrinfo", errmsg);
                                        ~~~~~~
liboping.c:206:2: note: 'snprintf' output between 14 and 269 bytes into
a destination of size 256
  snprintf (obj->errmsg, sizeof (obj->errmsg),
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    "%s: %s", function, message);

Fixes:
 - http://autobuild.buildroot.org/results/b12d86388b495a96194e0bcbb5c19a4e35cbc53d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>