You can not call :destroy() without :connect()

[vvzvlad]

Case:

tarantool> mqtt = require 'mqtt'
tarantool> mqtt_object = mqtt.new()

tarantool> mqtt_object:destroy()
---
- error: '...ocuments/Nokia/glue/.rocks/share/tarantool/mqtt/init.lua:472: attempt
    to index field ''fiber'' (a nil value)'
...

tarantool> mqtt_object:connect()
---
- true
- ok
...

tarantool> mqtt_object:destroy()
---
- true
- ok
...

[vvzvlad]

fix on #18 PR

[Vas Soshnikov]

Merged #18. Thanks!