env_nand: return error when no device is found

Currently, if there is an error probing the NAND chip and the env is based
in NAND, the readenv() function will use a NULL function pointer and thus
jump to address 0.

Here I just check for a non-zero value of blocksize as that shouldn't be
zero when a valid device is found, but perhaps there is a better way for
someone familiar with the NAND internals to suggest.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Tested-by: Ben Gardiner <bengardiner@nanometrics.ca>