Filip Roséen authored Mar 04, 2016 and Jean-Baptiste Kempf committed Mar 05, 2016

chapters.cpp:  `delete` is a NOOP if the operand is NULL, if-check removed.

chapters.{hpp,cpp}:
introduced helper for `chapter_item_c::{Enter,Leave}` since they
are extremely similar in their implementation.

The helper makes use of <algorithm> and <functional> to make the
code more error-proof, as a plus it is a little bit cleaner.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>