Fix RNC fail-open path and palette bounds compile error

[segrax]

Motivation

• A recent change caused recognized-but-invalid RNC resources to be returned as the original compressed buffer on validation or decompression failure, enabling downstream unbounded memcpy into fixed-size arrays and leading to memory corruption.
• The code also failed to build on some toolchains due to use of std::size on a raw array in Graphics_Amiga.cpp.

Description

• In Source/Resources.cpp added an InvalidRNC helper that returns an empty shared byte buffer and changed the header/size validation and rnc_unpack mismatch branches to return InvalidRNC() instead of pBuffer, so malformed or failed-to-decompress RNC resources produce an empty buffer rather than attacker-controlled compressed bytes.
• In Source/Amiga/Graphics_Amiga.cpp replaced std::size(Result.mPalette) with a portable capacity calculation sizeof(Result.mPalette) / sizeof(Result.mPalette[0]) before writing palette entries to fix the compile error.
• No other functional changes were introduced to resource loading callsites in this patch.

Testing

• Attempted to build the target with cmake --build . --target openfodder -j4, but the build could not be executed in this environment due to a missing CMake cache (Error: could not load cache).

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Codex Task