Fix debug assertion on packet length in blr_master.c

A debug assertion failed due to a NULL buffer but a non-zero packet
length. This was caused by a missing reset of the packet length after
freeing the buffers.

server/modules/routing/binlog/blr_master.c

@@ -1068,6 +1068,7 @@ blr_handle_binlog_record(ROUTER_INSTANCE *router, GWBUF *pkt)
 
                     gwbuf_free(pkt);
                     pkt = NULL;
+                    pkt_length = 0;
 
                     break;
                 }