contract invoke BytesN parsing fails to parse valid hex values

[leighmcculloch]

why "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef" succeed while "4142434400000000000000000000000000000000000000000000000000000000" fails? Both are properly formatted 64-chars hex strings representing 32 bytes.

Originally posted by @jsmaxi in #2239