[etherswitch] check if_alloc returns NULL

This patch is cosmetic. It checks if allocation of ifnet structure failed.
It's better to have this check rather than assume positive scenario.

Submitted b

[etherswitch] check if_alloc returns NULL

This patch is cosmetic. It checks if allocation of ifnet structure failed.
It's better to have this check rather than assume positive scenario.

Submitted by: Dmitry Luhtionov <[email protected]>
Reported by: Dmitry Luhtionov <[email protected]>

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