fw_env: rename variables holding env data content

Rename "addr0" and "addr1" to "buf0" and "buf1" accordingly. Name "addr"
suggests that variable contains a numeric value being some kind of
address. Name "buf" is de facto a standard name for pointer to allocated
memory for reading data to.

While at it drop redundant checks for NULL before calling free().

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>