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.