CVE-2015-5330: Fix handling of unicode near string endings

Until now next_codepoint_ext() and next_codepoint_handle_ext() were
using strnlen(str, 5) to determine how much string they should try to
decode. This ended up looking past the end of the string when it was not
null terminated and the final character looked like a multi-byte encoding.
The fix is to let the caller say how long the string can be.

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>
