fix possible NULL pointer use in vl.c

diff -p -u -r1.392 vl.c
@@ -985,6 +985,8 @@ QEMUTimer *qemu_new_timer(QEMUClock *clo
     QEMUTimer *ts;
 
     ts = qemu_mallocz(sizeof(QEMUTimer));
+    if (!ts)
+        return NULL;
     ts->clock = clock;
     ts->cb = cb;
     ts->opaque = opaque;