Event pipe: do not set file's thread_handler if not needed. This fixes a problem with aio threads and sendfile with aio_write switched off, as observed with range requests after fc72784b1f52 (1.9.13). Potential problems with sendfile in threads were previously described in 9fd738b85fad, and this seems to be one of them. The problem occurred as file's thread_handler was set to NULL by event pipe code after a sendfile thread task was scheduled. As a result, no sendfile completion code was executed, and the same buffer was additionally sent using non-threaded sendfile. Fix is to avoid modifying file's thread_handler if aio_write is switched off. Note that with "aio_write on" it is still possible that sendfile will use thread_handler as set by event pipe. This is believed to be safe though, as handlers used are compatible.