Unnecessary check of negative

[miurahr]

Hi, I found unnecessary check code in o.t.doublearray.DoubleArray and o.t.doublearray.TailDoubleArray``#getChild method.

   public DoubleArrayNode getChild(char c) {
            int code = charToCode[c];
            if (code == -1) return null;
            int nid = base[nodeId] + code;

Here charToCode is defined char[] and char is ranging from 0x0000 to 0xffff in language specification and never be negative.

I think it can be like

        public DoubleArrayNode getChild(char c) {
            int nid = base[nodeId] + charToCode[c];

I found it when I check the project by SpotBugs, the above code produce an error.

[takawitter]

Thanks again!
I modified the code and added some tests.