BUG/MINOR: spoe: Fix the message list splice with an empty list

BUG/MINOR: spoe: Fix the message list splice with an empty list

At the end of the parsing of a SPOE agent configuration file, the messages
of the file are moved to the agent message list with a manual splice.
However, this is only correct when the file defines at least one
"spoe-message" section. With an empty list, the manual splice will corrupt
the agent message list.

Replace the manual splice with LIST_SPLICE(), which does nothing with an
empty source list. This fixes the bug and this avoids to deal with list
pointers manually.

The regression was introduced by the commit 11610f3 ("MEDIUM: spoe: Parse
new "spoe-group" section in SPOE config file").

The patch must be backported to all supported versions.