BUF_strlcpy still assumes |src| is a NUL-terminated string and will call strlen
on it to determine the actual length. BUF_strndup's input need not be
NUL-terminated.

Change-Id: I9ca95e92533d12f1b0283412249bda4f8cf92433
Reviewed-on: https://boringssl-review.googlesource.com/1997
Reviewed-by: Adam Langley <agl@google.com>