perf evlist: Fix for double free in tools/perf stat

perf evlist: Fix for double free in tools/perf stat

Fix for double free bug in tools/perf due to dangling thread_map pointer
in perf_evlist struct.

Code path excercised when perf stat -C switch is used but not set and is
followed by another switch.

Example:

  perf stat -C -e.

Signed-off-by: Yasser Shalabi <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>