Fix: userspace-probe: unreported error on string copy error

Issue
=====

String copy errors, either due to the length or an allocation failure,
are not reported by
lttng_userspace_probe_location_tracepoint_create_from_payload
and don't log a clear error message.

This allowed truncation bugs like the one fixed in b45a296 to go
unnoticed.

Fix
===

Return an "invalid" status code and log a more descriptive error
message.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia07cac7cba315ea79337262e9082dd06eb60950f