fix IPV6NetworkMaskPrefixLength value parsing #1692
fix IPV6NetworkMaskPrefixLength value parsing #1692

Adapted from https://github.com/boot2docker/boot2docker-cli/commit/941c70cb0137acccd36c2f8ef9426a12ddc58ef0

Signed-off-by: Gianpaolo Macario <f8885408b8c3d931b330bae3c3a28a3af912c463@gmail.com>

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