bug#21953: Eliminate warnings in the emacs-25 release branch

[Eli Zaretskii]

> From: Andy Moreton <andrewjmoreton@gmail.com>
> Date: Wed, 25 Nov 2015 20:58:10 +0000
> 
> There are three in the mingw64 build (on emacs-25 or master branch):
> 
> ../../src/w32.c: In function 'sys_socket':
> ../../src/w32.c:7435:14: warning: overflow in implicit constant conversion 
> [-Woverflow]
>        return INVALID_SOCKET;
>               ^
> ../../src/w32.c: In function 'maybe_load_unicows_dll':
> ../../src/w32.c:9273:25: warning: assignment from incompatible pointer type 
> [-Wincompatible-pointer-types]
>     pMultiByteToWideChar = GetProcAddress (ret, "MultiByteToWideChar");
>                          ^
> ../../src/w32.c:9274:25: warning: assignment from incompatible pointer type 
> [-Wincompatible-pointer-types]
>     pWideCharToMultiByte = GetProcAddress (ret, "WideCharToMultiByte");
>                          ^

Thanks, fixed on emacs-25.