tests: fix segfault with no /etc/protocols (#286)

This file could be missing, which will cause getprotobynumber() to
return NULL.

The returned pointer is used to mention the protocol name in a warning
message, not critical. Instead, we can simply display 'Unknown' next to
the protocol ID.

Fixes: d6a48d4 ("tests: Expand mptcpwrap code coverage. (#169)")
Reported-by: Aurelien Jarno <[email protected]>
Closes: https://bugs.debian.org/1060285

Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

tests/mptcpwrap-tester.c

@@ -64,7 +64,7 @@ static void test_socket_data(struct socket_data const *data)
                                 "WARNING: Ignoring unsupported "
                                 "protocol: %d - %s\n",
                                 data->protocol,
-                                p->p_name);
+                                p ? p->p_name : "Unknown");
 
                         return;
                 }