[kwy404]

Fixes #4802

Root cause

With legacy support enabled, ZSTD_decompressStream() only recognized a v0.4 to v0.7 frame when a single call started with the magic number and carried at least 5 bytes. Two things broke a smaller first call:

1. When fewer than 5 bytes arrive, the bytes are buffered in headerBuffer and ZSTD_getFrameHeader_advanced() checks that they match a supported magic number. That check only knew the zstd and skippable magics, so a legacy prefix returned Unknown frame descriptor right away.
2. On the next call, the legacy detection ran ZSTD_isLegacy() on the current input instead of on the bytes already buffered in headerBuffer, so the magic was never seen there either.

Fix

• In the early magic check, also accept a prefix of a supported legacy magic.
• Detect the legacy frame from headerBuffer, replay those buffered bytes to the legacy stream decoder, then pass the rest of the input as before.

When the whole header comes in one call, the decoder gets the same bytes in the same order, so that path is unchanged.

Side effect: with legacy support compiled in, ZSTD_getFrameHeader() given 1 to 4 bytes of a legacy frame now asks for 5 bytes instead of returning prefix_unknown, as it already does for a zstd or skippable prefix. With 5 or more bytes it still returns an error for a legacy frame, as before.

Test

tests/legacy.c: testStreamingAPI() now takes a chunk size and is also run feeding the input one byte at a time, so every frame header is split across calls. The helper also stops calling ZSTD_initDStream() on every iteration (needsInit was never cleared), which would otherwise reset the stream between chunks.

Before the fix (MSVC build, ZSTD_LEGACY_SUPPORT=1):

Testing 8 frames, from format v0.1 upward
Simple API OK
Frame Decoding OK
Streaming API OK
ERROR: ZSTD_decompressStream: Unknown frame descriptor

After the fix:

Testing 8 frames, from format v0.1 upward
Simple API OK
Frame Decoding OK
Streaming API OK
Streaming API OK
OK

The reproducer from the issue now reports consistent for first calls of 1 to 8 bytes. zstreamtest -T30s and fuzzer -T30s (with ZSTD_LEGACY_SUPPORT=5) pass, and tests/legacy.c also passes with legacy support disabled.