Fix readwritesplit handling of unexpected responses

The backend server can send a response even if the client hasn't sent a
request. One case where this occurs is when the server is shutting
down. The internal logic of readwritesplit can't handle unexpected states
gracefully so the safest thing to do is to just ignore them and send the
responses to the client.

server/modules/protocol/MySQL/mysql_common.c

@@ -1498,23 +1498,18 @@ bool gw_read_backend_handshake(DCB *dcb, GWBUF *buffer)
     return rval;
 }
 
-/**
- * @brief Check if the buffer contains an OK packet
- *
- * @param buffer Buffer containing a complete MySQL packet
- * @return True if the buffer contains an OK packet
- */
 bool mxs_mysql_is_ok_packet(GWBUF *buffer)
 {
-    bool rval = false;
-    uint8_t cmd;
+    uint8_t cmd = 0xff; // Default should differ from the OK packet
+    gwbuf_copy_data(buffer, MYSQL_HEADER_LEN, 1, &cmd);
+    return cmd == MYSQL_REPLY_OK;
+}
 
-    if (gwbuf_copy_data(buffer, MYSQL_HEADER_LEN, 1, &cmd) &&  cmd == MYSQL_REPLY_OK)
-    {
-        rval = true;
-    }
-
-    return rval;
+bool mxs_mysql_is_err_packet(GWBUF *buffer)
+{
+    uint8_t cmd = 0x00; // Default should differ from the ERR packet
+    gwbuf_copy_data(buffer, MYSQL_HEADER_LEN, 1, &cmd);
+    return cmd == MYSQL_REPLY_ERR;
 }
 
 bool mxs_mysql_is_result_set(GWBUF *buffer)