cmd: nvedit: Validate argument count before use

cmd: nvedit: Validate argument count before use

Avoid NULL pointer dereference in case 'env select' is invoked
without parameters, check the arg count and make sure it is at
least 2, otherwise print usage.

The crash is easy to trigger e.g. in sandbox:
$ ./u-boot -Tc "env select"

Fixes: a97d22ebba23 ("cmd: env: add env select command")
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Tested-by: Vincent Stehlé <vincent.stehle@arm.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>