[LibOS] Fix an uninitialized value n in wait_event
Variable n is used without initialization.
If `if (!DkObjectsWaitAny(1, &e->event, NO_TIMEOUT))` condition is true, then code will skip the assignment of n with `n = DkStreamRead(e->event, 0, 1, &byte, NULL, 0);` and might finish the while loop. `n` should be properly initialized to prevent potential undefined behavior.
Signed-off-by: Gary <gang1.wang@intel.com>