perf symbols: Check if a struct machine instance was found

Which can happen when processing old files that had no fake kernel MMAP,
events.

That shouldn't result in perf_session__create_kernel_maps not being
called, this will be fixed in a followup patch, for now do these checks
to avoid segfaulting.

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/event.c

@@ -493,8 +493,10 @@ int event__process_mmap(event_t *self, struct perf_session *session)
 		return 0;
 	}
 
-	thread = perf_session__findnew(session, self->mmap.pid);
 	machine = perf_session__find_host_machine(session);
+	if (machine == NULL)
+		goto out_problem;
+	thread = perf_session__findnew(session, self->mmap.pid);
 	map = map__new(&machine->user_dsos, self->mmap.start,
 			self->mmap.len, self->mmap.pgoff,
 			self->mmap.pid, self->mmap.filename,
@@ -552,14 +554,18 @@ void thread__find_addr_map(struct thread *self,
 	if (cpumode == PERF_RECORD_MISC_KERNEL && perf_host) {
 		al->level = 'k';
 		machine = perf_session__find_host_machine(session);
+		if (machine == NULL) {
+			al->map = NULL;
+			return;
+		}
 		mg = &machine->kmaps;
 	} else if (cpumode == PERF_RECORD_MISC_USER && perf_host) {
 		al->level = '.';
 		machine = perf_session__find_host_machine(session);
 	} else if (cpumode == PERF_RECORD_MISC_GUEST_KERNEL && perf_guest) {
 		al->level = 'g';
 		machine = perf_session__find_machine(session, pid);
-		if (!machine) {
+		if (machine == NULL) {
 			al->map = NULL;
 			return;
 		}