BUG/MEDIUM: spoe: dup agent's engine_id string from trash.area

The agent's engine_id forgot to dup from trash, all engine_ids point to
the same address "&trash.area", the engine_id changed at run time and will
double free when release agents and trash.

This bug was introduced by the commit ee3bcddef ("MINOR: tools: add a generic
function to generate UUIDs").

No backport is needed, this is 2.2-dev.
diff --git a/src/flt_spoe.c b/src/flt_spoe.c
index df080d8..57c2246 100644
--- a/src/flt_spoe.c
+++ b/src/flt_spoe.c
@@ -258,7 +258,7 @@
 generate_pseudo_uuid()
 {
 	ha_generate_uuid(&trash);
-	return trash.area;
+	return my_strndup(trash.area, trash.data);
 }