virFirewallApply: Fix possible NULL dereference on error

Commit bbc25f0d03 juggled around some
error reporting. Unfortunately virFirewallApply tries to report the
errno stored in the firewall object and we'd try to do that when the
firewall object is NULL too. Report EINVAL if 'firewall' is NULL.

Found by Coverity.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

src/util/virfirewall.c

@@ -766,8 +766,12 @@ virFirewallApply(virFirewallPtr firewall)
         goto cleanup;
     }
     if (!firewall || firewall->err) {
-        virReportSystemError(firewall->err, "%s",
-                             _("Unable to create rule"));
+        int err = EINVAL;
+
+        if (firewall)
+            err = firewall->err;
+
+        virReportSystemError(err, "%s", _("Unable to create rule"));
         goto cleanup;
     }