pgsql: Fix off-by-one bug in bitncmp(): When comparing a number of bits

Log Message:
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Fix off-by-one bug in bitncmp(): When comparing a number of bits divisible by
8, bitncmp() may dereference a pointer one byte out of bounds.

Chris Mikkelson (bug #5101)

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/utils/adt:
        network.c (r1.56 -> r1.56.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/network.c?r1=1.56&r2=1.56.2.1)