perf script: Check sample->callchain before using it

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