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: NMike Frysinger <vapier@gentoo.org>
Tested-by: NBen Gardiner <bengardiner@nanometrics.ca>

common/env_nand.c

@@ -266,6 +266,8 @@ int readenv (size_t offset, u_char * buf)
 	u_char *char_ptr;
 
 	blocksize = nand_info[0].erasesize;
+	if (!blocksize)
+		return 1;
 	len = min(blocksize, CONFIG_ENV_SIZE);
 
 	while (amount_loaded < CONFIG_ENV_SIZE && offset < end) {