This patch adds a sanity check that avoids 'size' to overflow and crash when
importing an environment that contains a checksum. Example with the wrong size
that causes the crash:

=> env import -c 0x4100000 3 v1

This assumes that v1 has already been successfully exported with
'env export -c -s 0x100 0x4100000 v1'

Signed-off-by: Pedro Aguilar <pedro.aguilar@vimar.com>