mergedContext references incorrect index when waiting on a context pair to complete

[BarakatX2]

s3-sftp-proxy/merged_context.go

Lines 50 to 55 in d292467

	select {
	case <-ctxs.ctxs[0].Done():
	ctxs.err = ctxs.ctxs[0].Err()
	case <-ctxs.ctxs[1].Done():
	ctxs.err = ctxs.ctxs[0].Err()
	}

The second assignment incorrectly references the 0 index. This results in the context error being set to nil when it should never be set to nil. The next time the context is passed to another, it causes a panic that crashes the entire server:

panic: context: internal error: missing cancel error