Check if ai_list is null before freeing it (#831)

Passing null to freeaddrinfo is undefined behaviour. While glibc and uClibc both handle it safely,
musl does not, so a bad nodename or servname causes a segfault.

This change ensures that the ai_list is non-null before trying to free it. In musl and glibc this
condition will never be met, but there are some paths for it to be met in uClibc.

Author: micrictor

src/modbus-tcp.c

@@ -693,7 +693,9 @@ int modbus_tcp_pi_listen(modbus_t *ctx, int nb_connection)
             fprintf(stderr, "Error returned by getaddrinfo: %d\n", rc);
 #endif
         }
-        freeaddrinfo(ai_list);
+        if (ai_list != NULL) {
+            freeaddrinfo(ai_list);
+        }
         errno = ECONNREFUSED;
         return -1;
     }