common/cmd_nvedit.c: Fix size calculation in do_env_import()

do_env_import() missed the final '\0' terminator when calculating the
size of an environment data block.  This led to an erroneous 'bad CRC,
import failed' message for a checksum protected environment (-c.)

Signed-off-by: Horst Kronstorfer <hkronsto@frequentis.com>