input: avoid NULL dereference

Before using the result of env_get("stdin") we must check if it is NULL.

Avoid #if. This resolves the -Wunused-but-set-variable issue and we don't
need a dummy assignment in the else branch. Anyway this warning is
disabled in the Makefile.

For sake of readability use an early return after the configuration check.

Checking CONFIG_SPL_BUILD is incorrect as env_get() is only defined if
CONFIG_$(SPL_TPL)ENV_SUPPORT=y.

Fixes: 985ca394 ("spl: input: Allow input in SPL and TPL")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>