[src/core/jpg.js] Remove some redundant marker validation during the MCU parsing in the `decodeScan` function
[src/core/jpg.js] Remove some redundant marker validation during the MCU parsing in the `decodeScan` function

Some of the code in `src/core/jpg.js` is fairly old, and has with time become unnecessary when the surrounding code has been updated to handle various types of JPEG corruption.
In particular the `if (!marker || marker <= 0xff00) { ... }` branch is now dead code, since:

 - The `!marker` case can no longer happen, since we would already have broken out of the loop thanks to the `!fileMarker` branch a handful of lines above.

 - The `marker <= 0xff00` case can also no longer happen, since the `findNextFileMarker` function validate markers much more thoroughly (by checking `marker >= 0xffc0 && marker <= 0xfffe`). Hence we'd again have broken out of the loop via the `!fileMarker` branch above when no valid marker was found.

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