fix(socket): Fix return value of send and sendfile

The send function does not return an error when it sends fail.

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>

socket.c

@@ -501,6 +501,7 @@ static int lua_sendk(lua_State *L, int status, lua_KContext ctx)
             return lua_yieldk(L, 0, ctx, lua_sendk);
         }
 
+        lua_pushnil(L);
         if (errno == EPIPE)
             lua_pushliteral(L, "closed");
         else
@@ -569,7 +570,7 @@ static int lua_sendfilek(lua_State *L, int status, lua_KContext ctx)
         }
 
         close(sock->snd.fd);
-
+        lua_pushnil(L);
         if (errno == EPIPE)
             lua_pushliteral(L, "closed");
         else