Fix: consumerd(64/32)_lib_dir can be NULL

Reproducer:
lttng-sessiond \
    --consumerd32-path=/usr/local/lib/lttng/libexec/lttng-consumerd \
    --consumerd64-path=/usr/local/lib/lttng/libexec/lttng-consumerd

lttng create
lttng enable-event -u -a

On a 64bit machine the invocation of the 64bit consumerd will not fail
since its libdir is populated by sessiond_config_init but will segfault on
spawning of the 32 bit consumerd when performing the check of libdir
value.

On a 32bit machine the opposite will happen.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>