CVE-2015-5330: next_codepoint_handle_ext: don't short-circuit UTF16 low bytes

UTF16 contains zero bytes when it is encoding ASCII (for example), so we
can't assume the absense of the 0x80 bit means a one byte encoding. No
current callers use UTF16.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11599

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Pair-programmed-with: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
