perf mem: Free the allocated sort string, fixing a leak

The get_sort_order() returns either a new string (from strdup) or NULL
but it never gets freed.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Fixes: 2e7f545096f954a9 ("perf mem: Factor out a function to generate sort order")
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20240731235505.710436-3-namhyung@kernel.org
[ Added Fixes tag ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>