Backpatch Zdenek Kotala's fix to prevent pglz_decompress from stomping on

Backpatch Zdenek Kotala's fix to prevent pglz_decompress from stomping on
memory if the compressed data is corrupt.

Backpatch as far as 8.2.  The issue exists in older branches too, but given
the lack of field reports, it's not clear it's worth any additional effort
to adapt the patch to the slightly different code in older branches.