pgsql: Improve pglz_decompress() so that it cannot clobber memory beyond

Log Message:
-----------
Improve pglz_decompress() so that it cannot clobber memory beyond the
available output buffer when presented with corrupt input. Some testing
suggests that this slows the decompression loop about 1%, which seems an
acceptable price to pay for more robustness. (Curiously, the penalty
seems to be *less* on not-very-compressible data, which I didn't expect
since the overhead per output byte ought to be more in the literal-bytes
path.)

Patch from Zdenek Kotala. I fixed a corner case and did some renaming
of variables to make the routine more readable.

Modified Files:
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pgsql/src/backend/utils/adt:
pg_lzcompress.c (r1.30 -> r1.31)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pg_lzcompress.c?r1=1.30&r2=1.31)