Avoid race between sigchld and the tty master being closed

Don't break but continue if the master was closed.
This ensures that we always exit with the code from our child.

sshdrill.c

@@ -171,7 +171,7 @@ main(int argc, char *argv[])
 			if (cc == -1 && errno == EINTR)
 				continue;
 			if (cc <= 0)
-				break;
+				continue;
 			do_write(STDOUT_FILENO, obuf, cc);
 		}
 	}