fix problem when  ss_decrypt_all return NULL , server crash

server config
``` 
{
  "server" : "0.0.0.0",
  "server_port" : 6351,
  "password" : "somepassword",
  "method": "aes-256-cfb",
  "timeout": 600
}
```

run server with ` ./ss-server  -c config.json -u `

send some udp packets to the server 
```
echo -n "he" | nc -4u -q1 localhost 6351
```
server crashes with segment fault. 
```
(gdb) backtrace 
#0  0x00000000004078d9 in parse_udprealy_header (buf=0x0, buf_len=2, host=0x7fffffffe230 "", port=0x7fffffffe1f0 "")
    at udprelay.c:93
#1  0x0000000000408d4c in server_recv_cb (loop=0x621720 <default_loop_struct>, w=0x628320, revents=1) at udprelay.c:658
#2  0x00000000004137b2 in ev_invoke_pending (loop=0x621720 <default_loop_struct>) at ev.c:2994
#3  0x000000000041468c in ev_run (loop=0x621720 <default_loop_struct>, flags=0) at ev.c:3394
#4  0x000000000040f498 in main (argc=4, argv=0x7fffffffe658) at server.c:1064
```

It seems   when  server_recv_cb  is not checking  ss_decrypt_all return value , when it returns NULL , the server crash.