Fix FindBugs issues

src/main/java/org/apache/commons/collections4/ListUtils.java

@@ -673,9 +673,6 @@ public class ListUtils {
         @Override
         public List<T> get(final int index) {
             final int listSize = size();
-            if (listSize < 0) {
-                throw new IllegalArgumentException("negative size: " + listSize);
-            }
             if (index < 0) {
                 throw new IndexOutOfBoundsException("Index " + index + " must not be negative");
             }

src/main/java/org/apache/commons/collections4/list/TreeList.java

@@ -622,7 +622,7 @@ public class TreeList<E> extends AbstractList<E> {
             }
             if (getRightSubTree() == null) {
                 if (relativePosition > 0) {
-                    left.relativePosition += relativePosition + (relativePosition > 0 ? 0 : 1);
+                    left.relativePosition += relativePosition;
                 }
                 left.max().setRight(null, right);
                 return left;