If the transfer failed, the data might be invalid.

Change-Id: I3ad9daa00a54e2a3954983cec91b6685f1a98880
Found-By: Coverity Scan #1405870
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/40970


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>