Fix: Pass valid buffer to recv() instead of NULL

Valgrind reports:
  Syscall param socketcall.recvfrom(buf) points to unaddressable byte(s)
  at recv (recv.c:28)
  by xmppclient_end_session (xmppclient.c:57)
  Address 0x0 is not stack'd, malloc'd or (recently) free'd

The recv() call was passing NULL as buffer, which is invalid.
Now using a local char variable as discard buffer.

src/server/xmppclient.c

@@ -54,7 +54,7 @@ xmppclient_end_session(XMPPClient *client)
 
     if (client->sock) {
         shutdown(client->sock, 2);
-        while (recv(client->sock, NULL, 1, 0) > 0) {}
+        char discard_buf; while (recv(client->sock, &discard_buf, 1, 0) > 0) {}
         close(client->sock);
     }
     free(client->ip);