perf bench: Fix segfault at the end of an 'all' execution

At the end of

  $ perf bench all

the program segfaults because it attempts to dereference a NULL
pointer. Fix this fault.
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1395964219-22173-4-git-send-email-artagnon@gmail.comSigned-off-by: Jiri Olsa <jolsa@redhat.com>

tools/perf/builtin-bench.c

@@ -86,7 +86,7 @@ static struct collection collections[] = {
 
 /* Iterate over all benchmarks within a collection: */
 #define for_each_bench(coll, bench) \
-	for (bench = coll->benchmarks; bench->name; bench++)
+	for (bench = coll->benchmarks; bench && bench->name; bench++)
 
 static void dump_benchmarks(struct collection *coll)
 {