fusefrontend: doRead: skip decryption for an empty read

Previously we ran through the decryption steps even for an empty
ciphertext slice. The functions handle it correctly, but returning
early skips all the extra calls.

Speeds up the tar extract benchmark by about 4%.

internal/fusefrontend/file.go

@@ -181,6 +181,11 @@ func (f *file) doRead(dst []byte, off uint64, length uint64) ([]byte, fuse.Statu
 		tlog.Warn.Printf("read: ReadAt: %s", err.Error())
 		return nil, fuse.ToStatus(err)
 	}
+	// The ReadAt came back empty. We can skip all the decryption and return early.
+	if n == 0 {
+		f.fs.contentEnc.CReqPool.Put(ciphertext)
+		return dst, fuse.OK
+	}
 	// Truncate ciphertext buffer down to actually read bytes
 	ciphertext = ciphertext[0:n]