Fix PREFIX misparse (closes #854).

[RavuAlHemio]

Not skipping the leading '(' when splitting on the ')' always makes the modes list longer than the prefix list, which leads to an incorrect null return value.

[meebey]

Fix looks good but this needs some extra care:

            var prefixstr = RawProperties ["PREFIX"];
            if (prefixstr == null) {
                // supports no modes (!)
                return modesList;
            }

            // format: (modes)prefixes
            if (prefixstr [0] != '(') {
                return null;
            }

            var modesPrefixes = prefixstr.Substring(1).Split(')');

The issue is prefixstr can be an empty string or 1 character both woud lead to a IndexOutOfBoundsException, please add to the null check || prefixstr.Length < 2

[RavuAlHemio]

You're right -- and the class is really begging for unit tests anyway. If you could add a unit-test subproject to the SmartIrc4net solution, I'll write them.