avoid NegativeArraySizeException if input string is "\\#FFF#"

[kkolyan]

Forum Discussion: https://hub.jmonkeyengine.org/t/bitmaptext-colortags-negativearraysizeexception/43357

        // just for this micro-demo.
        BitmapFont font = new BitmapFont();
        font.setCharSet(new BitmapCharacterSet());
        font.setPages(new Material[0]);
        //  in game it's loaded with AssetManager, of course

        BitmapText text = new BitmapText(font);
        text.setText("\\#FFF#");

    Exception in thread "main" java.lang.NegativeArraySizeException
        at java.lang.AbstractStringBuilder.<init>(AbstractStringBuilder.java:68)
        at java.lang.StringBuilder.<init>(StringBuilder.java:101)
        at com.jme3.font.ColorTags.setText(ColorTags.java:80)
        at com.jme3.font.Letters.setText(Letters.java:68)
        at com.jme3.font.BitmapText.setText(BitmapText.java:181)

// ColorTags.java:80
StringBuilder builder = new StringBuilder(charSeq.length()-7);

Looks like it expects color tag in \#RGBA# (7 characters) and doesn't count that \#RGB# (6 characters) is valid too.

[stephengold]

Looks good to me. Thanks for your latest contribution!