config: fix check for environment string expansion

If ExpandEnvironmentStringsW is successful, it returns the amount of
characters written, including the NUL terminator.

Thanks to Emeric for reading the MSDN documentation correctly.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>

src/config.c

@@ -352,7 +352,7 @@ static int win32_find_system(char *system_config_path)
 		return GIT_ENOMEM;
 
 	ret = ExpandEnvironmentStringsW(query, apphome_utf16, size);
-	if (ret == 0 || ret >= size)
+	if (ret != size)
 		return git__throw(GIT_ERROR, "Failed to expand environment strings");
 
 	if (_waccess(apphome_utf16, F_OK) < 0) {