Fix: unchecked buffer size for communication header

Fix: unchecked buffer size for communication header

A number of object de-serialization functions rely on a
fixed-size communication header to create an object from
a payload.

A large number of those functions assume that the initial
header fits in the provided buffer or payload view. Also,
the functions that do validate that the header fits do so
in different ways:
  - checking the view's size,
  - creating a new fixed-size view and checking the 'data' pointer.

To harmonize all of those checks, the following utils are added:
  - lttng_buffer_view_is_valid()
  - lttng_payload_view_is_valid()

These functions should be used whenever a fixed-size view is
created (not passing -1 as the length parameter).

The checks are added and/or harmonized to:
  - create a new 'header' view,
  - validate it with the corresponding *_is_valid() function,
  - initialize the header pointer using the header view.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I763946feac714ecef4fc5bd427dab2d3fe5dc1a4