s3: fix crash if a bad listing is received - fixes #1927

Caringo Swarm is returning a listing with IsTruncated set but no
NextMarker and no Keys.  Rclone doesn't know how to continue the
listing at this point, so it returns an error rather than truncating
the listing or risking a loop.

s3/s3.go

@@ -554,6 +554,9 @@ func (f *Fs) list(dir string, recurse bool, fn listFn) error {
 		}
 		// Use NextMarker if set, otherwise use last Key
 		if resp.NextMarker == nil || *resp.NextMarker == "" {
+			if len(resp.Contents) == 0 {
+				return errors.New("s3 protocol error: received listing with IsTruncated set, no NextMarker and no Contents")
+			}
 			marker = resp.Contents[len(resp.Contents)-1].Key
 		} else {
 			marker = resp.NextMarker