Avoid a use-after-null-check in proto_socks.c

Coverity rightly complains that early in the function we're checking
whether username is NULL, and later we're passing it unconditionally
to strlen().

Fixes CID 1437967.  Bug not in any released Tor.

src/core/proto/proto_socks.c

@@ -166,7 +166,7 @@ parse_socks4_request(const uint8_t *raw_data, socks_request_t *req,
   *is_socks4a = (dest_ip >> 8) == 0;
 
   const char *username = socks4_client_request_get_username(trunnel_req);
-  size_t usernamelen = username ? strlen(username) : 0;
+  const size_t usernamelen = username ? strlen(username) : 0;
   if (username && usernamelen) {
     if (usernamelen > MAX_SOCKS_MESSAGE_LEN) {
       log_warn(LD_APP, "Socks4 user name too long; rejecting.");
@@ -184,7 +184,7 @@ parse_socks4_request(const uint8_t *raw_data, socks_request_t *req,
     // We cannot rely on trunnel here, as we want to detect if
     // we have abnormally long hostname field.
     const char *hostname = (char *)raw_data + SOCKS4_NETWORK_LEN +
-     strlen(username) + 1;
+     usernamelen + 1;
     size_t hostname_len = (char *)raw_data + datalen - hostname;
 
     if (hostname_len <= sizeof(req->address)) {