Second thoughts on TOAST decompression.

Second thoughts on TOAST decompression.

On detecting a corrupted match tag, pglz_decompress() should just
summarily return -1.  Breaking out of the loop, as I did in dfc797730,
doesn't quite guarantee that will happen.  Also, we can use
unlikely() on that check, just in case it helps.

Backpatch to v13, like the previous patch.