iso9660: Fix ../../ path normalization

iso9660: Fix ../../ path normalization

The function isofile_gen_utility_names could resolve .. directory
entries in a way that dirname will start with "../". If this happens,
the while-loop is unable to detect this because it forwards until the
cursor detects a slash again.

Fix this by also taking "../" at the beginning into account. Such an
entry can happen if "../../" points before the top directory.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>