perf script: Check if evsel has callchains before trying to use it

We were checking just if callchain processing was asked for by the
user, not if the evsel itself has callchains, and since we can have
some evsels with callchains and others without, check that.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-inxl7k49q9f9w1se039fbxuw@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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