perf symbols: Filter samples with unresolved symbol when "--symbols" option is used

Report/top commands support to only handle specific symbols with
"--symbols" option, but current code will keep those samples whose
symbol can't be resolved, which should actually be filtered.

If we run following commands:
$perf record -a tree
$perf report --symbols intel_idle -n
the output will be:

Without the patch:
==================
46.27% 156 sshd [unknown]
26.05% 48 swapper [kernel.kallsyms]
17.26% 38 tree libc-2.12.1.so
7.69% 17 tree tree
2.73% 6 tree ld-2.12.1.so

With the patch:
===============
100.00% 48 swapper [kernel.kallsyms]

Signed-off-by: Feng Tang <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/event.c

···
 		al->sym = map__find_symbol(al->map, al->addr, filter);
 	}
 
-	if (symbol_conf.sym_list && al->sym &&
-	    !strlist__has_entry(symbol_conf.sym_list, al->sym->name))
+	if (symbol_conf.sym_list &&
+		(!al->sym || !strlist__has_entry(symbol_conf.sym_list,
+						al->sym->name)))
 		goto out_filtered;
 
 	return 0;