perf hists browser: Only offer symbol scripting when a symbol is under the cursor

When this feature was introduced a check was made if there was a
resolved symbol under the cursor, it got lost in commit ea7cd592
("perf hists browser: Split popup menu actions - part 2"), reinstate it.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>,
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: ea7cd592 ("perf hists browser: Split popup menu actions - part 2")
Link: http://lkml.kernel.org/r/1452960197-5323-9-git-send-email-namhyung@kernel.org


[ Carved out from a  larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>