MT#55283 silence compiler warning

Some compilers seem to think that there is some uninitialised usage
here. Work around this.

Closes #1891

Change-Id: Ic97a4b589fd2a0c33418a209557b7ce29009c7bf
(cherry picked from commit 4b614d8)
(cherry picked from commit 83f078c)

daemon/nftables.c

@@ -389,7 +389,7 @@ static const char *add_rule(struct mnl_socket *nl, int family, uint32_t *seq,
 
 
 static const char *udp_filter(struct nftnl_rule *r, int family) {
-	g_autoptr(_nftnl_expr) e;
+	g_autoptr(_nftnl_expr) e = NULL;
 
 	static const uint8_t proto = IPPROTO_UDP;