firewall: Avoid creating a rule that permits all traffic on invalid source

The firewall engine generated rules that did not have any traffic
selectors due to an improperly initialized variable in the source.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Acked-by: Peter Müller <peter.mueller@ipfire.org>

config/firewall/rules.pl

@@ -401,6 +401,9 @@ sub buildrules {
 					$source = "";
 				}
 
+				# Make sure that $source is properly defined
+				next unless (defined $source);
+
 				my $source_intf = @$src[1];
 
 				foreach my $dst (@destinations) {