stand: libefi: avoid a null pointer deref in eficom

We don't keep comc_port around anymore if the console's not present, but
some things might still try to set one of the environment variables we
hook. In particular, one need not even set efi_com_port/efi_com_speed
in loader.conf; loader may do it itself and induce the crash if ConOut
depicts an available uart.

Probably reported by: dch
OK for now: imp