bgpd: bgp_attr_flags_diagnose shouldn't assert

* bgpd/bgp_attr.c: (bgp_attr_flags_diagnose) debug code for error-handling
  paths probably shouldn't assert, instead it should just log that there
  was no problem.
diff --git a/bgpd/bgp_attr.c b/bgpd/bgp_attr.c
index c21655c..0d82aba 100644
--- a/bgpd/bgp_attr.c
+++ b/bgpd/bgp_attr.c
@@ -809,7 +809,14 @@
             attr_flag_str[i].str);
       seen = 1;
     }
-  assert (seen);
+  if (!seen)
+    {
+      zlog (args->peer->log, LOG_DEBUG,
+            "Strange, %s called for attr %s, but no problem found with flags"
+            " (real flags 0x%x, desired 0x%x)",
+            __func__, LOOKUP (attr_str, attr_code),
+            real_flags, desired_flags);
+    }
 }
 
 /* Required flags for attributes. EXTLEN will be masked off when testing,