Fix SIGFPE in ZSTD_estimateCCtxSize_usingCCtxParams when LDM is enabled

[fresh3nough]

Summary

Fixes #4590

ZSTD_estimateCCtxSize_usingCCtxParams and ZSTD_estimateCStreamSize_usingCCtxParams raise a SIGFPE (division by zero) on x86 when long distance matching (LDM) is enabled via ZSTD_CCtxParams_setParameter but the individual LDM sub-parameters (hashLog, bucketSizeLog, minMatchLength, hashRateLog) remain at their default zero values.

The division by zero occurs in ZSTD_ldm_getMaxNbSeq() at lib/compress/zstd_ldm.c:181, where maxChunkSize / params.minMatchLength is computed with minMatchLength == 0.

Root Cause

When the user enables LDM through ZSTD_CCtxParams_setParameter(params, ZSTD_c_enableLongDistanceMatching, 1), only the enableLdm flag is set. The other LDM parameters stay at zero. The existing ZSTD_estimateCCtxSize_usingCParams path avoids this by going through ZSTD_makeCCtxParamsFromCParams, which calls ZSTD_ldm_adjustParameters() to fill in sensible defaults. However, ZSTD_estimateCCtxSize_usingCCtxParams and ZSTD_estimateCStreamSize_usingCCtxParams skip this adjustment step and pass the raw (zeroed) LDM params directly to the internal estimation function.

Fix

Both ZSTD_estimateCCtxSize_usingCCtxParams and ZSTD_estimateCStreamSize_usingCCtxParams now copy the LDM parameters to a local and call ZSTD_ldm_adjustParameters() when LDM is enabled, consistent with the pattern already used in ZSTD_makeCCtxParamsFromCParams.

Changes

• lib/compress/zstd_compress.c: Added LDM parameter adjustment in both estimation functions
• tests/fuzzer.c: Added regression test that enables LDM via CCtxParams and calls both estimation functions

Steps to Reproduce (before fix)

#define ZSTD_STATIC_LINKING_ONLY
#include <zstd.h>
#include <stdio.h>
#include <stdlib.h>

int main(void) {
    ZSTD_CCtx_params* p = ZSTD_createCCtxParams();
    ZSTD_CCtxParams_setParameter(p, ZSTD_c_compressionLevel, 22);
    ZSTD_CCtxParams_setParameter(p, ZSTD_c_enableLongDistanceMatching, 1);
    size_t r = ZSTD_estimateCCtxSize_usingCCtxParams(p);
    /* SIGFPE on x86, incorrect result on ARM */
    printf("estimated CCtx size: %zu\n", r);
    ZSTD_freeCCtxParams(p);
    return 0;
}

Compile and run:

cc -g -I lib repro.c lib/libzstd.a -o repro && ./repro

Testing

• Verified the reproducer no longer crashes and returns a correct estimation
• make check passes
• make -C tests fuzzer && tests/fuzzer -t1 passes including the new regression test

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