libsystemd-network: Avoid potential NULL dereference in test-lldp

As a malloc0 could fail, doing a strncpy without checking could
cause issues. Adding an assert should be good enough and in line
with other similar routines in the code.

Found with Coverity Fixes: CID#1261402

src/libsystemd-network/test-lldp.c

@@ -166,6 +166,7 @@ static int lldp_parse_system_name_tlv(tlv_packet *m) {
         assert_se(tlv_packet_read_string(m, &str, &length) >= 0);
 
         p = malloc0(length + 1);
+        assert_se(p);
         strncpy(p, str, length);
 
         assert_se(streq(p, TEST_LLDP_TYPE_SYSTEM_NAME) == 1);