perf script: Check sample->callchain before using it

Found by code inspection, while looking at thread__resolve_callchain()
callsites, one had it, the other didn't.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-6r8i2afd3523thuuaxl39yhk@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-script.c

@@ -791,7 +791,7 @@ static void process_event(struct perf_script *script,
 	if (PRINT_FIELD(IP)) {
 		struct callchain_cursor *cursor = NULL, cursor_callchain;
 
-		if (symbol_conf.use_callchain &&
+		if (symbol_conf.use_callchain && sample->callchain &&
 		    thread__resolve_callchain(al->thread, &cursor_callchain, evsel,
 					      sample, NULL, NULL, scripting_max_stack) == 0)
 			cursor = &cursor_callchain;