pgsql: Make bit/varbit substring() treat any negative length as meaning

Log Message:
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Make bit/varbit substring() treat any negative length as meaning "all the rest
of the string". The previous coding treated only -1 that way, and would
produce an invalid result value for other negative values.

We ought to fix it so that 2-parameter bit substring() is a different C
function and the 3-parameter form throws error for negative length, but
that takes a pg_proc change which is impractical in the back branches;
and in any case somebody might be relying on -1 working this way.
So just do this as a back-patchable fix.

Modified Files:
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pgsql/src/backend/utils/adt:
varbit.c (r1.61 -> r1.62)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varbit.c?r1=1.61&r2=1.62)