internal/stream: reject sizes below one tag in EncryptedChunkCount

[sweis]

EncryptedChunkCount returned (0, nil) for negative or sub-tag-size inputs —
the consistency check rounds both sides to 0 and passes silently. This would
cascade into NewDecryptReaderAt computing finalChunkIndex = -1.

Fix: a valid payload always carries at least one Poly1305 tag, even for empty
plaintext. Reject anything smaller than chacha20poly1305.Overhead up front.

Test: go test ./internal/stream/ -run TestEncryptedChunkCountRejectsSmallSizes -v

[FiloSottile]

This looks just wrong: I get invalid encrypted payload size: 0 and invalid encrypted payload size: 15 from the current code, not nil.