[Chieler]

Fixes #4843

When decompressing a malformed frame header using a single-use prefix (ZSTD_DCtx_refPrefix), decompression aborts before ZSTD_getDDict() can consume the dictionary. A subsequent ZSTD_DCtx_reset(session_only) reset streamStage back to zdss_init without clearing dictUses == ZSTD_use_once, allowing the discarded prefix buffer to leak into subsequent frames and causing heap use-after-free if the caller freed the prefix buffer.

Clear the prefix during session reset if a frame decompression was already initiated (dctx->streamStage != zdss_init). Add regression tests in zstreamtest to verify that the prefix is dropped after a bad header and preserved if reset occurs prior to ingesting input.

This change clears the prefix during session reset if a frame decompression was already initiated (dctx->streamStage != zdss_init).

Regression tests have been added to tests/zstreamtest.c to verify that:

1. The prefix is dropped after a bad frame header is rejected.
2. The prefix is preserved if reset occurs prior to ingesting any input.

Checklist

• [ x] I searched open pull requests and issues: this problem is not already being addressed.
• [x ] 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.
  A gain in one configuration is not a general gain: see the Performance section of CONTRIBUTING.md.
• [ x] I stated below whether AI or other automated tools were used.

Tools used

Byteask was used for root cause diagnosis, implementing the fix in zstd_decompress.c, and writing the regression tests in zstreamtest.c.