[MINOR] Use DPRINTF in assign_server()

Use DPRINTF in assign_server() rather than open-coding its logic.
diff --git a/src/backend.c b/src/backend.c
index 2cbc888..31e29d3 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -471,9 +471,7 @@
 	struct server *srv, *prev_srv;
 	int err;
 
-#ifdef DEBUG_FULL
-	fprintf(stderr,"assign_server : s=%p\n",s);
-#endif
+	DPRINTF(stderr,"assign_server : s=%p\n",s);
 
 	err = SRV_STATUS_INTERNAL;
 	if (unlikely(s->pend_pos || s->flags & SN_ASSIGNED))