wsd: do not complain if process is reaped already

When a process is already reaped, waitpid(3) will
fail. Instead of blindly warning, we first check
if the process or its zombie still exists.
If neither is found, there is nothing to do,
and therefore nothing to warn about.

Change-Id: I4d2d35a58eb3cfe6b70b0d2593e62f4ee1015581
Signed-off-by: Ashod Nakashian <ashod.nakashian@collabora.co.uk>

common/SigUtil.cpp

@@ -179,7 +179,7 @@ void requestShutdown()
                 return std::make_pair(ret, WTERMSIG(status));
             }
         }
-        else if (pid > 0)
+        else if (pid > 0 && ::kill(pid, 0) == 0) // Don't complain if the process is reaped already.
         {
             // Log errno if we had a child pid we expected to reap.
             LOG_WRN("Failed to reap child process " << pid << " (" << Util::symbolicErrno(errno)