decompress: make ZSTD_decompressBound() a true upper bound

[ilyas-mallah]

What and why

ZSTD_decompressBound() is documented as an upper bound on the decompressed size, but two kinds of input make it return less than ZSTD_decompress() produces. The decoder still honors dstCapacity, so a caller that allocates the bound gets dstSize_tooSmall on input that otherwise decodes.

1. Overflow. Frame content sizes are summed without an overflow check, so two frames that each declare 2^63 bytes give a bound of 0. ZSTD_findDecompressedSize() already guards the same sum, and ZSTD_decompressBound() now does too.
2. Oversized Raw and RLE blocks. Single-pass decoding accepts Raw and RLE blocks larger than blockSizeMax (spec: 0-sized compressed blocks in frames with window_size = 0 #3482), but the bound counts every block as blockSizeMax. A 10-byte frame with one RLE block decodes to 17,597 bytes against a bound of 6,144. Such blocks now count at their declared size, and valid frames keep the same bound.

The other way to fix the second case is to reject these blocks in ZSTD_decompressFrame(), as streaming does and as the spec says (#4669). That changes which inputs decode, so this PR keeps them decodable and only fixes the bound. I can switch to rejecting them if that's preferred.

A new unit test fails on dev, and two fuzz targets now check the bound, skipping legacy frames since the frozen legacy decoders don't keep it. tests/fuzzer (64-bit and 32-bit), test-fuzzer and test-legacy pass, and 300 s per fuzz target with ASan and UBSan found nothing.

Checklist

• I searched open pull requests and issues: this problem is not already being addressed.
• Bug fix: it comes with a reproducer or a test that fails without the fix.
• Performance change: numbers from repeated runs are included, with CPU, compiler, flags and data.
• I stated below whether AI or other automated tools were used.

Tools used

AI usage: Claude Code helped with the fuzzing runs and test builds. I traced both cases, made the fix, and validated every change.

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