usb: sandbox: Check for string end in copy_to_unicode()

When copying the string in copy_to_unicode(), check for the null
terminator in each position, not just at the start, to avoid reading
beyond the end of the string.

Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>