Ensure that grub_reboot doesn't return on emu.

Use grub_fatal if longjmp fails.

grub_reboot is marked as noreturn so return would cause
a crash.

grub-core/kern/emu/main.c

@@ -63,6 +63,7 @@ void
 grub_reboot (void)
 {
   longjmp (main_env, 1);
+  grub_fatal ("longjmp failed");
 }
 
 void