common: env_ubi: Clear environment buffer before reading

common: env_ubi: Clear environment buffer before reading

In case we have restarted u-boot there is a chance that environment
buffer contains old environment (from the previous boot). If UBI volume
is zero size, ubi_volume_read() doesn't modify the buffer and exits
successfully.

We need to clear buffer manually before reading it from UBI, so the
invalid CRC will cause setting default environment in case that the UBI
volume is zero size.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>