pgsql: Fix heap-buffer-overflow in pglz_decompress() on corrupt input.

Fix heap-buffer-overflow in pglz_decompress() on corrupt input.

When decoding a match tag, pglz_decompress() reads 2 bytes (or 3
for extended-length matches) from the source buffer before checking
whether enough data remains. The existing bounds check (sp > srcend)
occurs after the reads, so truncated compressed data that ends
mid-tag causes a read past the allocated buffer.

Fix by validating that sufficient source bytes are available before
reading each part of the match tag. The post-read sp > srcend
check is no longer needed and is removed.

Found by fuzz testing with libFuzzer and AddressSanitizer.

Backpatch-through: 14

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c88ad3a2122eae875b77eb5cba3b7bda5c92f251

Modified Files
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src/common/pg_lzcompress.c | 27 +++++++++++++++++++--------
1 file changed, 19 insertions(+), 8 deletions(-)