Panic when generating Micro QR

When I try to generate a Micro QR code, the library panics.

Example:

```rust
use qrcode::{EcLevel, QrCode, Version};

fn main() {
    let code = QrCode::with_version("123123123123A", Version::Micro(3), EcLevel::M).unwrap();
}
```

```
attempt to subtract with overflow
```