ima: fix invalid memory reference

commit 7b7e5916 upstream.

Don't free a valid measurement entry on TPM PCR extend failure.

Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>