Fix: tracker: NULL pointer dereference after NULL check

Fix: tracker: NULL pointer dereference after NULL check

value_view can be NULL and must thus be checked before use.

Moreover, the fix introduced in 1ad5cb59 is erreneous: the
function must validate that either:
  - value is a 'name' type, value_view is not null, and not len == 0,
  - value is an integer and value_view does not contain more data.

In process_attr_value_from_comm: Pointer is checked against null but
then dereferenced anyway (CWE-476)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia130ef57e10118960f1023338b90f7a10d588ee2