common: env_nand: Ensure that we have nand_info[0] prior to use

Now that nand_info[] is an array of pointers we need to ensure that it's
been populated prior to use.  We may for example have ENV in NAND set in
configurations that run on boards with and without NAND (where default
env is fine enough, such as omap3_beagle and beagleboard (NAND) vs
beagle xM (no NAND)).

Fixes: b616d9b0 ("nand: Embed mtd_info in struct nand_chip")
Cc: Scott Wood <oss@buserror.net>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Scott Wood <oss@buserror.net>