Fix occasional decoder errors decoding trace data collected in snapshot
mode.

Snapshot mode can take successive snapshots of trace which might overlap.
The decoder checks whether there is an overlap but only looks at the
current and previous buffer. However buffers that do not contain
synchronization (i.e. PSB) packets cannot be decoded or used for overlap
checking. That means the decoder actually needs to check overlaps between
the current buffer and the previous buffer that contained usable data.
Make that change.
Signed-off-by: NAdrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: stable@vger.kernel.org # v4.3+
Link: http://lkml.kernel.org/r/1474641528-18776-10-git-send-email-adrian.hunter@intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>