test: check return value of write()/send() in bufferevent_read_failed

test: check return value of write()/send() in bufferevent_read_failed

This will also suppress:

    ../test/regress_bufferevent.c: In function ‘test_bufferevent_read_failed’: 1102
    ../test/regress_bufferevent.c:1395:2: warning: ignoring return value of ‘write’, declared with attribute warn_unused_result [-Wunused-result] 1103
      write(data->pair[0], buf, strlen(buf));