env: Make _init() expect _INVALID when _IS_NOWHERE

Avoid applying the "fix" introduced by commit 5557eec0 ("env: Fix
invalid env handling in env_init()") to the environment "nowhere".

This is necessary as that commit, by setting the return value of
env_init() to -ENOENT if gd->env_valid is ENV_INVALID, forces that
function to reset gd->env_valid to ENV_VALID. By doing so, it breaks the
assumption (required by ENV_IS_NOWHERE) that gd->env_valid must be
ENV_INVALID.

This, in turn, results in env_relocate() calling env_load() (it should
not), which itself, calls U_BOOT_ENV_LOCATION(nowhere).load() i.e.
env_nowhere_load(). That function, being implemented under the
assumption mentioned above, calls env_set_default(), which in turn,
seeing that gd->env_valid is ENV_VALID (it should not), tries to
dereference whatever lies in gd->env_addr (most likely garbage), leading
to a faulty memory access.

Note that other env_locations might be concerned by this bug but that
this commit only intends to fix it for when ENV_IS_NOWHERE.

Fixes: 5557eec0 ("env: Fix invalid env handling in env_init()")
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>