restore: print a error if ptrace() failed

CID 85039 (#1 of 1): Unchecked return value (CHECKED_RETURN)
6. check_return: Calling ptrace without checking return value (as is done elsewhere 44 out of 49 times).
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/cr-restore.c

@@ -1361,7 +1361,8 @@ static void sigchld_handler(int signal, siginfo_t *siginfo, void *data)
 		if (!current && WIFSTOPPED(status) &&
 					WSTOPSIG(status) == SIGCHLD) {
 			/* The root task is ptraced. Allow it to handle SIGCHLD */
-			ptrace(PTRACE_CONT, siginfo->si_pid, 0, SIGCHLD);
+			if (ptrace(PTRACE_CONT, pid, 0, SIGCHLD))
+				pr_perror("Unable to resume %d", pid);
 			return;
 		}