libipt: fix standalone mode.tsx handling

When branch tracing is disabled, MODE.TSX is sent standalone.  We model this
by suppressing the IP in the ptev_tsx event.

The latter was missing.  Fix it.

Change-Id: I04ae3fb77c8cd097210b2e4af4d6101ba43dc0cb
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>