Fix: sessiond: user/group name can be leaked on malformed command

process_attr_value_from_comm() can leak a copy of the user/group
name when the value type is erroneous. This is not reachable in
"normal" execution, but could be triggered by invalid "crafter"
lttng-ctl commands.

In process_attr_value_from_comm: Leak of memory or pointers to
system resources (CWE-404).

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I7ef55c0743c954a93e3d27ce17e6478708b49437