pgsql: Fix combo_decrypt() to throw an error for zero-length input when

Log Message:
-----------
Fix combo_decrypt() to throw an error for zero-length input when using a
padded encryption scheme. Formerly it would try to access res[(unsigned) -1],
which resulted in core dumps on 64-bit machines, and was certainly trouble
waiting to happen on 32-bit machines (though in at least the known case
it was harmless because that byte would be overwritten after return).
Per report from Ken Colson; fix by Marko Kreen.

Modified Files:
--------------
pgsql/contrib/pgcrypto:
px.c (r1.15 -> r1.16)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/px.c?r1=1.15&r2=1.16)
px.h (r1.17 -> r1.18)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/px.h?r1=1.17&r2=1.18)