Clean-up: sessiond: silence negative index warning

Coverity warns that `lttng_action_get_type()` can return
a negative index (LTTNG_ACTION_TYPE_UNKNOWN). This scenario
is not reachable, but a check is added to silence the analyzer.

Original report:
  1435955 Negative array index read

  A memory location at a negative offset from the beginning of the array
  will be read, resulting in incorrect values.

  In get_action_name: Negative value used to index an array in a read
  operation (CWE-129)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5952096a1d29f0d4a3c4350a2a842874d5f3973b