nios2: time: Read timer in get_cycles only if initialized

Mainline crashes as follows when running nios2 images.

On node 0 totalpages: 65536
free_area_init_node: node 0, pgdat c8408fa0, node_mem_map c8726000
  Normal zone: 512 pages used for memmap
  Normal zone: 0 pages reserved
  Normal zone: 65536 pages, LIFO batch:15
Unable to handle kernel NULL pointer dereference at virtual address 00000000
ea = c8003cb0, ra = c81cbf40, cause = 15
Kernel panic - not syncing: Oops

Problem is seen because get_cycles() is called before the timer it depends
on is initialized. Returning 0 in that situation fixes the problem.

Fixes: 33d72f38 ("init/main.c: extract early boot entropy from the ..")
Cc: Laura Abbott <labbott@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Daniel Micay <danielmicay@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

arch/nios2/kernel/time.c

@@ -107,7 +107,10 @@ static struct nios2_clocksource nios2_cs = {
 
 cycles_t get_cycles(void)
 {
+	/* Only read timer if it has been initialized */
+	if (nios2_cs.timer.base)
 		return nios2_timer_read(&nios2_cs.cs);
+	return 0;
 }
 EXPORT_SYMBOL(get_cycles);