tape: Fix "n" size returned by DecodeGBEU

tape/types.go

@@ -113,10 +113,10 @@ func DecodeGBEU[T UInt](data []byte) (value T, n int, err error) {
 		fullValue *= 0x80
 		fullValue += uint64(chunk & 0x7F)
 		ccb := chunk >> 7
+		n += 1
 		if ccb == 0 {
 			return T(fullValue), n, nil
 		}
-		n += 1
 	}
 	return 0, n, fmt.Errorf("decoding GBEU: %w", ErrGBEUNotTerminated)
 }