Do not destroy vif's napi context, if it was not initialised during addition

If the napi structure was not initialised during interface addition, because
of errors,  it should not be touched during the delete too, since doing a
netif_napi_del results in crash. However, there are no reliable checks to find
whether napi was initialised or not. Hence, for now we will check for the existance
poll function, which should have been set if the napi structure was initialised.

Closes BUG: #1399577

Change-Id: I8cf439dc53805801a5ba301f542dedb2aaa5dee2