Fixes issues in sys_semaphore_create() and sys_event_flag_(create/get)()

`sys_semaphore_create()`
In case of att.addr() being NULL,
CELL_EFAULT must be returned and not CELL_EINVAL,
according to the corresponding ps3autotest.

`sys_event_flag_(create/get)()`
Neither sys_event_flag_create() nor sys_event_flag_get() checked for
NULL pointers in their arguments, which caused the corresponding test
from ps3autotests to segfault.