NetworkPkg: better sanity check on Ipv6 prefix length
NetworkPkg: better sanity check on Ipv6 prefix length

Fix a possible buffer overrun issue that could occur if PrefixLength >
128 . Changed == 128 to >= 128. Also remove check for Byte < 16, which
is no longer possible because of the first change.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Samer El-Haj-Mahmoud <69075e1ba2600094d95dfeb0aa01056bdaba053c@hpe.com>
Reviewed-by: Fu Siyuan <eef549df2036636f79f66102b4e2dc444721df8f@intel.com>

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