MXS-1785: Don't assume empty packet is for LDLI

When a LOAD DATA LOCAL INFILE finishes, the client sends an empty
packet. The second case when the client sends an empty packet when the
previous packet was exactly 0xffffff bytes long. These two packets were
confused which caused the internal state to temporarily flip from inactive
to ending and back to inactive.

The aforementioned flip-flopping didn't have any practical differences but
it was caught by a debug assertion.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -168,7 +168,7 @@ route_target_t get_target_type(RWSplitSession *rses, GWBUF *buffer,
             route_target = get_route_target(rses, *command, *type, buffer->hint);
         }
     }
-    else
+    else if (rses->load_data_state == LOAD_DATA_ACTIVE)
     {
         /** Empty packet signals end of LOAD DATA LOCAL INFILE, send it to master*/
         rses->load_data_state = LOAD_DATA_END;