STAP: Only print transaction statistics if the script caught some transactions
If the script measured an 'id' run from the cache, there would be no
transactions and dereferencing the aggrefate would throw an error.
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>
(cherry picked from commit 150a0cc8fe1936002af136e5552ef6cdd210956f)