iso9660: Fix OOB in Joliet ID generation

iso9660: Fix OOB in Joliet ID generation

Allocate enough memory for possible addition of 3 characters within the
range of 0-Z. Since UTF-16 is in use, allocate 6 bytes + 2 bytes for the
terminating NUL character.

Also keep in mind that "l" is already size in bytes, which means that a
multiplication of 2 is not needed (and prevented overflow issues with
longer filenames).

Resolves #2935.

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