Fix: command reply message is leaked for variable-len replies

Fix: command reply message is leaked for variable-len replies

Commands which return a variable-length payload re-setup the
command context using setup_lttng_msg() (and its wrappers).

In doing so, the lttcomm_lttng_msg structure (plus its trailing
variable-length payload) are re-allocated. However, the previous
instance of lttcomm_lttng_msg is leaked.

This is solved by free()-ing the original lttcomm_lttng_msg when
setup_lttng_msg() is used. When it is only used once, a NULL
pointer will be free'd without any effect.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>