perf session: Free environment information when deleting session

The perf session environment information was saved (so allocated) during
perf_session__open, but was not freed.  As free(3) handles NULL pointer
input properly it won't cause a issue for writing modes - e.g. perf
record

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1353472999-23042-1-git-send-email-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>