ima: fix error handling logic when file measurement failed

Restore the error handling logic so that when file measurement fails,
the respective iint entry is not left with the digest data being
populated with zeroes.

Fixes: 54f03916 ("ima: permit fsverity's file digests in the IMA measurement list")
Cc: stable@vger.kernel.org	# 5.19
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>