Volume: validate the "indexInVolume" arguments

Author: stephengold

src/main/java/com/github/stephengold/joltjni/Volume.java

@@ -82,6 +82,8 @@ public void setSixRestVolume(float sixVolume) {
      * @param indexInMesh the index of the vertex to assign (≥0)
      */
     public void setVertex(int indexInVolume, int indexInMesh) {
+        assert indexInVolume >= 0 && indexInVolume < 4 :
+                "indexInVolume = " + indexInVolume;
         long volumeVa = va();
         setVertex(volumeVa, indexInVolume, indexInMesh);
     }
@@ -125,6 +127,8 @@ public float getSixRestVolume() {
      */
     @Override
     public int getVertex(int indexInVolume) {
+        assert indexInVolume >= 0 && indexInVolume < 4 :
+                "indexInVolume = " + indexInVolume;
         long volumeVa = va();
         int result = getVertex(volumeVa, indexInVolume);