Allow alarm_cancel() on NULL timers

Remove an assert that prevents using alarm_cancel() with NULL
timers.

Bug: 26831803
Change-Id: Ifd1476272b13a3f7fd5a0dbb57ee867cbfe2c2bb
diff --git a/system/osi/src/alarm.c b/system/osi/src/alarm.c
index a00aacc..2ccdd4f 100644
--- a/system/osi/src/alarm.c
+++ b/system/osi/src/alarm.c
@@ -260,7 +260,8 @@
 
 void alarm_cancel(alarm_t *alarm) {
   assert(alarms != NULL);
-  assert(alarm != NULL);
+  if (!alarm)
+    return;
 
   pthread_mutex_lock(&monitor);