ovsdb: Fix Coverity leak warning by marking code as unreachable.

Coverity reports a memory leak on the 'error' variable in
ovsdb_trigger_try(). However, this code path is unreachable due to an
ovs_assert() in an earlier function call.

To make this clear to Coverity and silence the warning, the section is
explicitly marked as unreachable.

Acked-by: Mike Pattrick <mkp@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>

ovsdb/trigger.c

@@ -405,6 +405,8 @@ ovsdb_trigger_try(struct ovsdb_trigger *t, long long int now)
                     jsonrpc_msg_destroy(t->reply);
                     t->reply = NULL;
                     trigger_convert_error(t, error);
+                } else {
+                    OVS_NOT_REACHED();
                 }
             }
         } else {