oprofile: fix uninitialized use of struct op_entry

Impact: fix crash

In case of losing samples struct op_entry could have been used
uninitialized causing e.g. a wrong preemption count or NULL pointer
access. This patch fixes this.

Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/drivers/oprofile/cpu_buffer.c b/drivers/oprofile/cpu_buffer.c
index 2e03b6d..e76d715 100644
--- a/drivers/oprofile/cpu_buffer.c
+++ b/drivers/oprofile/cpu_buffer.c
@@ -393,16 +393,21 @@
 	return;
 
 fail:
+	entry->event = NULL;
 	cpu_buf->sample_lost_overflow++;
 }
 
 int oprofile_add_data(struct op_entry *entry, unsigned long val)
 {
+	if (!entry->event)
+		return 0;
 	return op_cpu_buffer_add_data(entry, val);
 }
 
 int oprofile_write_commit(struct op_entry *entry)
 {
+	if (!entry->event)
+		return -EINVAL;
 	return op_cpu_buffer_write_commit(entry);
 }