fixup! mingw: Windows Docker volumes are *not* symbolic links

A quick fix: upon success, readlink() returns a non-negative value (not
0, as the code expected).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -3566,7 +3566,7 @@ int file_attr_to_st_mode (DWORD attr, DWORD tag, const char *path)
 		 * points and look like symbolic links, but they are not.
 		 */
 		if (path && is_inside_windows_container() &&
-		    !readlink(path, buf, sizeof(buf)) &&
+		    !readlink(path, buf, sizeof(buf) >= 0) &&
 		    starts_with(buf, "/ContainerMappedDirectories/"))
 			flag = S_IFDIR;