fix(NetworkUtils): guard formatNetmask against an undefined mask

formatNetmask() uses its first argument immediately (inet_aton($mask),
2**$mask, hex $mask) with no check that it is defined. Callers that pass an
undefined mask trigger "Use of uninitialized value" warnings and a
meaningless result instead of a clean failure.

Return undef up front when $mask is not defined.

Recovered from the unmerged lenovobuild branch (original acbbeb86).

Co-authored-by: Jarrod Johnson <10814490+jjohnson42@users.noreply.github.com>

perl-xCAT/xCAT/NetworkUtils.pm

@@ -1389,6 +1389,7 @@ sub formatNetmask
     my $origType = shift;
     my $newType  = shift;
     my $maskn;
+    if (not defined $mask) { return undef; }
     if ($origType == 0)
     {
         $maskn = unpack("N", inet_aton($mask));