srt: Abort write when epoll fails

Copied from https://gitlab.freedesktop.org//gstreamer/gst-plugins-bad/-/merge_requests/1767

When a caller-mode srtsink gets flushed, the socket is removed from the poll. This results in gst_srt_object_write_one entering an endless loop as srt_epoll_wait invariably fails.

Do the same handling gst_srt_object_read does and abort the write in this case; don't return -1 as that will trigger an element error.