qrexec: added two I/O error checks, even though it is redundant in these cases

qrexec/qrexec_daemon.c

@@ -98,6 +98,11 @@ void init(int xid)
 	umask(0007); 		// make the log readable by the "qubes" group
 	logfd = open(qrexec_error_log_name, O_WRONLY | O_CREAT | O_TRUNC, 0640);
 
+	if (logfd < 0) {
+	        perror("open");
+	        exit(1);
+        }
+
 	dup2(logfd, 1);
 	dup2(logfd, 2);
 

qrexec/write_stdin.c

@@ -49,7 +49,10 @@ int flush_client_data(int fd, int client_id, struct buffer *buffer)
 			} else
 				return WRITE_STDIN_BUFFERED;
 		}
-		buffer_remove(buffer, len);
+		// we previously called buffer_remove(buffer, len)
+		// it will be wrong if we change MAX_DATA_CHUNK to something large
+		// as pipes writes are atomic only to PIPE_MAX limit 
+		buffer_remove(buffer, ret);
 		len = buffer_len(buffer);
 		if (!len) {
 			struct server_header s_hdr;