perf script: hook up perf_scripting_context->pevent

Running the check-perf-trace scripts causes segfaults in both the Perl
and Python cases:

  # perf script record check-perf-trace
  # perf script -s libexec/perf-core/scripts/python/check-perf-trace.py
  trace_begin
  Segmentation fault (core dumped)

The reason is that the 'pevent' field was added to
perf_scripting_context but it wasn't hooked up with an actual pevent in
either case, so when one of the 'common' fields is accessed (in
util/trace-event-parse.c:get_common_fields()), pevent->events tries to
dereference a NULL pointer.

This sets the pevent field when the scripting context is set up.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Link: http://lkml.kernel.org/r/d2b1b8166a6ca0a36e1f5255b88a8289058ba236.1358527965.git.tom.zanussi@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>