add sanity check for siginfo to ensure whether there is really a consumer waiting for the signal, since the task state will not be changed appropriately if in cancel/killed case Test Case: https://github.com/linux-test-project/ltp/blob/master/testcases/open_posix_testsuite/conformance/interfaces/sigpause/1-1.c#L63 Signed-off-by: chao an <anchao@lixiang.com>