perf probe: Use zfree() to avoid possibly accessing dangling pointers

When freeing a->b it is good practice to set a->b to NULL using
zfree(&a->b) so that when we have a bug where a reference to a freed 'a'
pointer is kept somewhere, we can more quickly cause a segfault if some
code tries to use a->b.

Convert one such case in the 'perf probe' codebase.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/ZjpBnkL2wO3QJa5W@x1Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-probe.c

@@ -325,7 +325,7 @@ static void cleanup_params(void)
 	for (i = 0; i < params->nevents; i++)
 		clear_perf_probe_event(params->events + i);
 	line_range__clear(&params->line_range);
-	free(params->target);
+	zfree(&params->target);
 	strfilter__delete(params->filter);
 	nsinfo__put(params->nsi);
 	zfree(&params);