mingw: fix possible buffer overrun when calling GetUserNameW()

In 39a98e9 (mingw: get pw_name in UTF-8 format, 2019-06-27), this
developer missed the fact that the `GetUserNameW()` function takes the
number of characters as `len` parameter, not the number of bytes.

Reported-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -1952,7 +1952,7 @@ struct passwd *getpwuid(int uid)
 	if (initialized)
 		return p;
 
-	len = sizeof(buf);
+	len = ARRAY_SIZE(buf);
 	if (!GetUserNameW(buf, &len)) {
 		initialized = 1;
 		return NULL;