get-mac-address with position 0 tries to return loop back address

[Takashi Kato]

The following always returns #vu8(0 0 0 0 0 0) on Linux (or not Cygwin environment POSIX).

(get-mac-address)

Seems it's getting loop back address.

[Takashi Kato]

Changing style a bit for Sg_GetMacAddress(Fixing #90)

→ <<cset b493059cff8d>>