Fix: lttng-ctl: appending to dynamic buffer invalidates its data member

Fix: lttng-ctl: appending to dynamic buffer invalidates its data member

`lttng_register_trigger` samples the address of the lsm header in the
message payload. However, it does so before calling
`lttng_trigger_serialize()` which may increase the underlying buffer's
size (and cause a realloc()).

Most of the time the message buffer is large enough _or_ its realloc
yields the same address which hid the problem. However, I stumbled on
a case (a trigger which snapshots to a long location) where the realloc
ends-up returning a completely different address, causing invalid data
to be sent to the session daemon.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I8e4323dac778bc2a1af7b6e2cca42f6521abaee2