perf bench: Fix NULL pointer dereference in "perf bench all"

The for_each_bench() macro must check that the "benchmarks" field of a
collection is not NULL before dereferencing it because the "all"
collection in particular has a NULL "benchmarks" field (signifying that
it has no benchmarks to iterate over).

This fixes this NULL pointer dereference when running "perf bench all":

  [root@ssdandy ~]# perf bench all
  <SNIP>

  # Running mem/memset benchmark...
  # Copying 1MB Bytes ...

         2.453675 GB/Sec
        12.056327 GB/Sec (with prefault)

  Segmentation fault (core dumped)
  [root@ssdandy ~]#

Signed-off-by: Patrick Palka <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: Patrick Palka

tools/perf/builtin-bench.c

@@ -76,7 +76,7 @@ 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)
 {