win32: rewrite getcwd() using GetFullPathNameW

_wgetcwd is apparently not available in all runtimes. Well, whatever.

osdep/io.c

@@ -337,9 +337,18 @@ int mp_mkdir(const char *path, int mode)
 
 char *mp_win32_getcwd(char *buf, size_t size)
 {
-    wchar_t *wres = _wgetcwd(NULL, 0);
-    if (!wres)
+    if (size >= SIZE_MAX / 3 - 1) {
+        errno = ENOMEM;
         return NULL;
+    }
+    size_t wbuffer = size * 3 + 1;
+    wchar_t *wres = talloc_array(NULL, wchar_t, wbuffer);
+    DWORD wlen = GetFullPathNameW(L".", wbuffer, wres, NULL);
+    if (wlen >= wbuffer || wlen == 0) {
+        talloc_free(wres);
+        errno = wlen ? ERANGE : ENOENT;
+        return NULL;
+    }
     char *t = mp_to_utf8(NULL, wres);
     free(wres);
     size_t st = strlen(t);