Fix: sessiond: NULL pointer dereference after NULL check

The process attribute value deserialization allows the buffer view to
be NULL when the value's type is not USER_NAME nor GROUP_NAME. This is
not checked when ensuring that no string is passed (len == 0) in the
case of integral values.

A NULL check is added to the condition.

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I343f747c325f739196284dadd3c407cfb4084268