genl_family: explicitly null terminate strncpy destination buffer

genl_family: explicitly null terminate strncpy destination buffer

The strncpy() function doesn't null terminate the destination string if
the source string is at least as long as the destination. (This behavior
is defined by the C99 specification.) As a result, the destination
string must be null terminated after calling strncpy().

And clang11 static analyzer thus reports following:

 genl_family.c:148:2: error: ‘strncpy’ output may be truncated copying 15 bytes from a string of length 15 [-Werror=stringop-truncation]
   148 |  strncpy(grp->name, name, GENL_NAMSIZ - 1);
       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

References: https://gitlab.com/openwrt/project/libnl-tiny/-/jobs/2495301251#L197
Signed-off-by: Petr Štetiar <ynezz@true.cz>