perf kvm: fix segfault with default guest arguments

File generated with:

$ perf kvm --host --guest --guestkallsyms=/tmp/guest-kallsyms
    --guestmodules=/tmp/guest-modules record -a

The report command:

$ perf kvm --host --guest --guestkallsyms=/tmp/guest-kallsyms
    --guestmodules=/tmp/guest-modules report

dies with a SEGFAULT:
Program received signal SIGSEGV, Segmentation fault.
0x000000000046dd7b in machine__mmap_name (self=0x0, bf=0x7fffffffbd20 "q\021", size=4096) at util/map.c:715
715 snprintf(bf, size, "[%s.%d]", "guest.kernel.kallsyms", self->pid);

(gdb) bt

    at util/event.c:668
    file_offset=8480) at util/session.c:979
    0x7fffffffdd80) at util/session.c:1363

If no samples hit within a guest module then no build id events are written to the header during the record. On the report side this cases no 'machine' to be generated for the default guest (pid == 0) leading to a machine == NULL in perf_session_deliver_event.

perf's handling of default kallsyms and guest-modules needs to be reworked. Until then perf should at least not segfault.

---
 tools/perf/util/session.c |    2 ++
 1 file changed, 2 insertions(+)

index c3e399b..3999d5e 100644
@@ -976,6 +976,8 @@ static int perf_session_deliver_event(struct perf_session *session,
 		}
 		return tool->sample(tool, event, sample, evsel, machine);
 	case PERF_RECORD_MMAP:
+		if (machine == NULL)
+			return 0;
 		return tool->mmap(tool, event, sample, machine);
 	case PERF_RECORD_COMM:
 		return tool->comm(tool, event, sample, machine);

1.7.10.1