libipt, insn: avoid spurious commit indications

When looking at longer traces with ptxed I sometimes got spurious commits,
even though there were no transactions in the workload.

Strengthen the check for commits to see if there was actually a transaction
before.

Change-Id: Iab55863b2bdc32dcadc91d784476a3110c38aeb1
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>